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
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