Most Vulkan drivers are happy to compile a pipeline with interstage
interface mismatches, even if the pipeline will be broken. Metal,
and therefore MoltenVK, instead doesn't and errors out immediately.
This is required to properly optimize signatures, because these
semantics must be alligned while being packed:
- Array elements.
- The first fields of structs.
- Major vectors of a matrix.
For now this has no effect since semantics are allocated with reg_size
4, but will have effect when optimizing interstage signatures.
The generated pixel shader input signature must be consistent with the
generated vertex shader output signature for the same data type.
Since the interpolation mode affects allocation order, the allocator
needs to know the modifiers for both input and output signature elements.
The descriptor heap implementation is a rather central behavior element
in vkd3d, so it's useful to have all the relevant information logged
in a single place.
Vulkan doesn't mandate whether sampling exactly in the middle between
two levels should resolve to one or the other, while D3D specifies
that it should result into sampling the higher level. llvmpipe
happens to choose the lower one instead, at least in some cases.
