Specifically, add flattening of arrayed io for geometry and
tesselation shaders. Previously some interface structs just had
builtins split out which caused some interfaces to not be exactly
the same as that of flattened adjacent stages, affecting validation
and correctness.
This obviates builtin splitting. That will be removed in a followup
commit. It was left in for this commit to better exhibit the functional
changes that were made.
Fixes#1660.
The goal is to flatten all I/O, but there are multiple categories and
steps to complete, likely including a final unification of splitting
and flattening.
HLSL requires vec2 tessellation coordinate declarations in some cases
(e.g, isoline topology), where SPIR-V requires the TessCoord qualified
builtin to be a vec3 in all cases. This alters the IO form of the
variable to be a vec3, which will be copied to the shader's declared
type if needed. This is not a validation; the shader type must be correct.
