Request: Add a Cap for ScalarBlockLayout etc

[kusma]

In Vulkan 1.2, the scalarBlockLayout feature was added (as well as the related uniformBufferStandardLayout feature), which allows struct-members inside uniform, SSBO et al blocks to be aligned to the natural alignment instead of 16.

Unfortunately, there's no SPIR-V cap that encodes that this is expected in a shader, which means that the spirv-val tool have to make assumptions. It currently assumes that the capability is not supported, and emits a warning on tightly packed scalars.

It would IMO be much better if we could have SPIR-V caps to encode these requirements, so it's clearly visible in the shader without having to parse types etc what rules apply. There's a related feature called workgroupMemoryExplicitLayoutScalarBlockLayout that probably have the same issue.

I realize that this can only be added to newer versions of SPIR-V, we can't retroactively add it to old versions. But I think adding it to future versions would be a win.