No Vulkan equivalent to DXGI_FORMAT_A8_UNORM

Vulkan has no equivalent to `DXGI_FORMAT_A8_UNORM`. The closest format is `VK_FORMAT_R8_UNORM` but it can't be used to fully emulate `DXGI_FORMAT_A8_UNORM` because swizzled image views can't be used for storage image descriptors.

The issue concerns DXGI_FORMAT_A8_UNORM, VK_FORMAT_R8_UNORM, swizzled image views, and storage image descriptors in the Vulkan specification. Start by reviewing the relevant format and descriptor documentation, then determine whether an equivalent or specification change is required. Done means the compatibility gap has a documented resolution or an accepted Vulkan design.

Written by the indexing model from the issue text.