Crash on startup when only llvmpipe (CPU Vulkan) is available instead of graceful error

Nobody has claimed this yet.

When launching Graphite on a system without a Vulkan-capable GPU, the application crashes during startup instead of failing gracefully.

Graphite selects the Vulkan CPU adapter (llvmpipe) and proceeds with initialization, which later fails with a wgpu validation error and eventually results in a panic and segmentation fault.

This makes it difficult for users to understand that their system does not meet the graphics requirements.

**Environment**

- OS: Fedora Linux 43 Workstation
- Architecture: x86_64
- Machine type: Physical machine
- Graphite build: Official AppImage (downloaded from Graphite Discord)
- Launch method: Running the AppImage directly in terminal (./Graphite.AppImage)

**Graphics setup**

_Integrated GPU: Intel 4 Series Chipset Integrated Graphics Controller_
- OpenGL 2.1 (Core / Compatibility)
- No Vulkan support

_Vulkan fallback:_
- Vulkan CPU (llvmpipe)
- Mesa version: 25.2.7 (LLVM 21.1.5)

No Vulkan-capable GPU is detected, so the CPU adapter is used by default.

**Steps to reproduce**

1. Run Graphite on a system where Vulkan only exposes a CPU adapter (llvmpipe)
2. Launch the official Graphite AppImage
3. Observe the crash during startup

**Actual behavior**

- Vulkan / EGL initialization fails
- wgpu reports a shader validation error
- The application panics and crashes with a segmentation fault

Relevant log excerpt:

wgpu error: Validation Error

Shader validation error:
Shader requires capability SHADER_FLOAT16_IN_FLOAT32

Adapter: Cpu (llvmpipe)

thread 'main' panicked at wgpu_core.rs:1058

**Expected behavior**

Graphite should detect early that the selected graphics adapter (llvmpipe / CPU Vulkan)
does not support required shader capabilities (e.g., float16),
and exit gracefully with a clear, user-facing error message.

_Example:_

"Your system does not support the required GPU features to run Graphite.
A compatible GPU with Vulkan support is required."

**Additional context**

- The system has an integrated Intel 4 Series GPU, which is too old to support Vulkan.
- As a result, Vulkan falls back to the llvmpipe CPU adapter.
- This report is not requesting support for this hardware, but highlights that Graphite currently crashes instead of clearly reporting unsupported GPU capabilities.

Planned future hardware: AMD Radeon RX 550, which should meet the required GPU capabilities.

**Full Log**

```
Available WGPU adapters:
[0] Vulkan Cpu (Name: llvmpipe (LLVM 21.1.5, 128 bits), Driver: llvmpipe, Device: 0)

Using WGPU adapter: AdapterInfo { name: "llvmpipe (LLVM 21.1.5, 128 bits)", vendor: 65541, device: 0, device_type: Cpu, driver: "llvmpipe", driver_info: "Mesa 25.2.7 (LLVM 21.1.5)", backend: Vulkan }
MESA-LOADER: failed to open dri: /run/opengl-driver/lib/gbm/dri_gbm.so: cannot open shared object file: No such file or directory (search paths /run/opengl-driver/lib/gbm, suffix _gbm)
MESA-LOADER: failed to open dri: /run/opengl-driver/lib/gbm/dri_gbm.so: cannot open shared object file: No such file or directory (search paths /run/opengl-driver/lib/gbm, suffix _gbm)
MESA-LOADER: failed to open dri: /run/opengl-driver/lib/gbm/dri_gbm.so: cannot open shared object file: No such file or directory (search paths /run/opengl-driver/lib/gbm, suffix _gbm)
[11709:11709:0101/121625.965188:ERROR:ui/gl/angle_platform_impl.cc:42] Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
ERR: Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11709:11709:0101/121625.965390:ERROR:ui/gl/egl_util.cc:92] EGL Driver message (Critical) eglInitialize: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11709:11709:0101/121625.965460:ERROR:ui/gl/gl_display.cc:639] eglInitialize Vulkan failed with error EGL_NOT_INITIALIZED
[11709:11709:0101/121625.965542:ERROR:ui/gl/gl_display.cc:674] Initialization of all EGL display types failed.
[11709:11709:0101/121625.965603:ERROR:ui/ozone/common/gl_ozone_egl.cc:26] GLDisplayEGL::Initialize failed.
[11709:11709:0101/121625.970990:ERROR:ui/gl/angle_platform_impl.cc:42] Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
ERR: Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11709:11709:0101/121625.971172:ERROR:ui/gl/egl_util.cc:92] EGL Driver message (Critical) eglInitialize: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11709:11709:0101/121625.971242:ERROR:ui/gl/gl_display.cc:639] eglInitialize Vulkan failed with error EGL_NOT_INITIALIZED
[11709:11709:0101/121625.971317:ERROR:ui/gl/gl_display.cc:674] Initialization of all EGL display types failed.
[11709:11709:0101/121625.971452:ERROR:ui/ozone/common/gl_ozone_egl.cc:26] GLDisplayEGL::Initialize failed.
[11709:11709:0101/121625.994385:ERROR:components/viz/service/main/viz_main_impl.cc:189] Exiting GPU process due to errors during initialization
MESA-LOADER: failed to open dri: /run/opengl-driver/lib/gbm/dri_gbm.so: cannot open shared object file: No such file or directory (search paths /run/opengl-driver/lib/gbm, suffix _gbm)
[11741:11741:0101/121626.397907:ERROR:ui/gl/angle_platform_impl.cc:42] Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
ERR: Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11741:11741:0101/121626.398103:ERROR:ui/gl/egl_util.cc:92] EGL Driver message (Critical) eglInitialize: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11741:11741:0101/121626.398185:ERROR:ui/gl/gl_display.cc:639] eglInitialize Vulkan failed with error EGL_NOT_INITIALIZED
[11741:11741:0101/121626.398260:ERROR:ui/gl/gl_display.cc:674] Initialization of all EGL display types failed.
[11741:11741:0101/121626.398326:ERROR:ui/ozone/common/gl_ozone_egl.cc:26] GLDisplayEGL::Initialize failed.
[11741:11741:0101/121626.408324:ERROR:ui/gl/angle_platform_impl.cc:42] Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
ERR: Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11741:11741:0101/121626.408484:ERROR:ui/gl/egl_util.cc:92] EGL Driver message (Critical) eglInitialize: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11741:11741:0101/121626.408557:ERROR:ui/gl/gl_display.cc:639] eglInitialize Vulkan failed with error EGL_NOT_INITIALIZED
[11741:11741:0101/121626.408620:ERROR:ui/gl/gl_display.cc:674] Initialization of all EGL display types failed.
[11741:11741:0101/121626.408679:ERROR:ui/ozone/common/gl_ozone_egl.cc:26] GLDisplayEGL::Initialize failed.
[11741:11741:0101/121626.410569:ERROR:components/viz/service/main/viz_main_impl.cc:189] Exiting GPU process due to errors during initialization
2026-01-01T15:16:26.613085Z ERROR wgpu::backend::wgpu_core: Handling wgpu errors as fatal by default

thread 'main' (11667) panicked at /nix/store/jyn2a0vyd3yy00mgdhkwgyq7skz2hd21-vendor-cargo-deps/c19b7c6f923b580ac259164a89f2577984ad5ab09ee9d583b888f934adbbe8d0/wgpu-27.0.1/src/backend/wgpu_core.rs:1058:30:
wgpu error: Validation Error

Caused by:
In Device::create_shader_module, label = 'vello.flatten'

Shader validation error: Function [33] 'draw_join' is invalid
┌─ vello.flatten:823:1
│
823 │ ╭ fn draw_join(
824 │ │ path_ix: u32, style_flags: u32, p0: vec2f,
825 │ │ tan_prev: vec2f, tan_next: vec2f,
826 │ │ n_prev: vec2f, n_next: vec2f,
· │
843 │ │ let miter_limit = unpack2x16float(style_flags & STYLE_MITER_LIMIT_MASK)[0];
│ │ ^^^^^^^^^^^^^^^ naga::ir::Expression [30]
· │
890 │ │ default: {}
891 │ │ }
│ ╰─────^ naga::ir::Function [33]
│
= Expression [30] is invalid
= Shader requires capability Capabilities(SHADER_FLOAT16_IN_FLOAT32)

Expression [30] is invalid
Shader requires capability Capabilities(SHADER_FLOAT16_IN_FLOAT32)

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
MESA-LOADER: failed to open dri: /run/opengl-driver/lib/gbm/dri_gbm.so: cannot open shared object file: No such file or directory (search paths /run/opengl-driver/lib/gbm, suffix _gbm)
[11764:11764:0101/121626.688974:ERROR:ui/gl/angle_platform_impl.cc:42] Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
ERR: Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11764:11764:0101/121626.689505:ERROR:ui/gl/egl_util.cc:92] EGL Driver message (Critical) eglInitialize: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11764:11764:0101/121626.689683:ERROR:ui/gl/gl_display.cc:639] eglInitialize Vulkan failed with error EGL_NOT_INITIALIZED
[11764:11764:0101/121626.689921:ERROR:ui/gl/gl_display.cc:674] Initialization of all EGL display types failed.
[11764:11764:0101/121626.690012:ERROR:ui/ozone/common/gl_ozone_egl.cc:26] GLDisplayEGL::Initialize failed.
[11764:11764:0100/000000.701494:ERROR:ui/gl/angle_platform_impl.cc:42] Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
ERR: Display.cpp:1093 (initialize): ANGLE Display::initialize error 0: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11764:11764:0100/000000.466466:ERROR:ui/gl/egl_util.cc:92] EGL Driver message (Critical) eglInitialize: Internal Vulkan error (-3): Initialization of an object could not be completed for implementation-specific reasons, in ../../third_party/angle/src/libANGLE/renderer/vulkan/vk_renderer.cpp, initialize:2276.
[11764:11764:0100/000000.466556:ERROR:ui/gl/gl_display.cc:639] eglInitialize Vulkan failed with error EGL_NOT_INITIALIZED
[11764:11764:0100/000000.466622:ERROR:ui/gl/gl_display.cc:674] Initialization of all EGL display types failed.
[11764:11764:0100/000000.466684:ERROR:ui/ozone/common/gl_ozone_egl.cc:26] GLDisplayEGL::Initialize failed.
Falha de segmentação (imagem do núcleo gravada) ./Graphite.AppImage
```

The log identifies wgpu_core.rs:1058 and the vello.flatten shader-validation path; begin by tracing startup adapter selection and device initialization around those errors. Reproduce with a Vulkan-only CPU adapter, then verify that unsupported required features produce a clear user-facing exit instead of a panic or segmentation fault.

Written by the indexing model from the issue text.