Fix OpImageRead result type when compiling HLSL
- Per the Vulkan spec, OpImageRead must return a 4-component vector always. When compiling HLSL, loads from a RWTexture of a template type with < 4 components would incorrectly generate an OpImageRead with a < 4 component result, resulting in validation errors.
- This was previously fixed for OpImageFetch in commit 4425f24; this commit does the same thing for OpImageRead.
- Added associated tests and expanded existing image fetch tests to check all the different types of textures, in both float and int incarnations, for completeness.
- Update other HLSL tests involving OpImageRead
This needs some render testing, but is destined to be part of master.
This also leads to a variety of other simplifications.
- IO are global symbols, so only need one list of linkage nodes (deferred)
- no longer need parse-context-wide 'inEntryPoint' state, entry-point is localized
- several parts of splitting/flattening are now localized
This PR adds:
1. The "u" register class for RW* objects.
2. --shift-image-bindings (== --sib), analogous to --shift-texture-bindings etc.
3. Case insensitive reg classes.
4. Tests for above.
