This turned out to be a little bit more involved than I had hoped. But,
with this, we can now use the `VK_FORMAT_A4R4G4B4_UNORM_PACK16` and
`VK_FORMAT_A4B4G4R4_UNORM_PACK16` formats from shaders, use them as blit
sources, and even clear them. Storage images and render targets of these
formats aren't supported, however. To support the latter would require
the insertion of a swizzle into the fragment shader before returning.
The former cannot be reasonably supported.
5d173d0e09