Vulkan GLSL: uniform block with push_constant allocation

[tangent-vector]

Both Vulkan and D3D12 share the idea that "push constants" ("root constants") are exposed to the shading language as ordinary uniform blocks (cbuffers). In D3D12 the actual assignment of a buffer to root constant resources is handled at the API level, while in Vulkan all of the binding to API-level resources is handled in layout modifiers.

A GLSL uniform block decorated with layout(push_constant):

layout(push_constant) uniform MyStuff { ... };

consumes different resources from an ordinary uniform block. This needs to be accounted for in a few places:

• We need to parse the push_constant layout modifier into a form visible to semantics
• The ParameterBinding.cpp and/or TypeLayout.cpp logic needs to allocate resources differently for these blocks
  • Don't allocate a descriptor table slot for the block
  • Allocate a new resource kind (PushConstant) for members of the block

The existing reflection API should be able to handle this case.

Actually, one parting shot: it might be worthwhile to go ahead and detect this modifier during parsing, so that we can break out a distinct type for the push_constant block like what we do for in and out blocks already. That would probably simplify a lot of the logic because the block would no longer surface as an "ordinary" uniform block.