So that when mutable descriptors are in use we can avoid putting
the other descriptor sets backing the SRV-UAV-CBV descriptor heap
in the pipeline layout altogether.
So we avoid hardcoding that it is number zero. There are two
goals here: first making the code easier to understand and
second allow reshuffling the descriptor set allocation in a
later commit.
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.
