Running with -vulkan only shows a RGB triangle

i'm not sure if this is expected because the Vulkan renderer is still experimental, but i couldn't find it documented anywhere: running with `-vulkan` only shows a full screen RGB triangle.

It does produce sound and seems to respond to input.

This is on Ubuntu Linux, AMD graphics card w/ amdgpu drivers:
```
Device Properties and Extensions:
=================================
GPU0:
VkPhysicalDeviceProperties:
---------------------------
apiVersion = 1.3.274 (4206866)
driverVersion = 24.0.9 (100663305)
vendorID = 0x1002
deviceID = 0x699f
deviceType = PHYSICAL_DEVICE_TYPE_DISCRETE_GPU
deviceName = AMD Radeon RX 550 / 550 Series (RADV POLARIS12)
pipelineCacheUUID = 604006ad-74c5-5c83-34d8-a903358c60d4
```
OpenGL mode works fine on this system.

Version: commit 228fd17ff12b31d4fbe61031aecd7c6d10969383 built from source