We only support the `robustImageAccess2` feature for now. Metal's
guarantees around out-of-bounds accesses to textures give us this for
free. `nullDescriptor` should also be possible to implement, but it
needs testing. Null image descriptors will probably just work, but null
buffer pointers probably not. We may need help, either from SPIRV-Cross
or a pass in SPIRV-Tools. `robustBufferAccess2` definitely needs an
assist from SPIRV-Tools. All three are useful for Direct3D compatibility
layers (DXVK, wined3d).
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