vk/image: Document a Vulkan spec requirement for depthstencil
vk/image: Document a Vulkan spec requirement for depthstencil

The Vulkan spec (git a511ba2) requires support for some combined depth
stencil formats.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler