Matrix of the form `matCx2` not getting translated properly when in a uniform buffer

[teoxoy]

WGSL expects a matCx2 to have a matrix stride of 8 (inner vector components to be aligned to 8 bytes including itself).
SPIR-V/GLSL/HLSL expect a matCx2 to have a matrix stride of 16 (inner vector components to be aligned to 16 bytes including itself).

Solution

Replace matCx2 with a sequence of C vec2's (since vec2s alignment is 8) and reconstruct the matrix in the shader.

References

WGSL Alignment and Size
GLSL Standard Uniform Block Layout
HLSL DirectXShaderCompiler Buffer-Packing Wiki
Vulkan Offset and Stride Assignment

Related issues

gfx-rs/naga#1400 (gfx-rs/naga#1400 (comment)), #4375, #4376 - inverse of this issue (concerning the frontends)

[teoxoy]

Reopened since the PR only solved the issue in the HLSL backend. The GLSL backend still needs changes.

[teoxoy]

We'll have to do this for spv as well... See gfx-rs/naga#2034.