Initial snapshot tests

[fornwall]

Part of #7.

Setup skeleton for snapshot tests:

• tests/snapshots/in: Contains .wgsl, .glsl or .spvasm files as input.
• tests/snapshots/in-spvasm: Contains the .spv files generated from the above .wgsl (using naga) or .glsl (using glslangValidator`) in files.
  • Not strictly necessary to have checked in, but might be good to track changes across wgsl and glsl compilers.
• tests/snapshots/out-spirt: The .spirt files we generate from the files in the previous step.
• tests/snapshots/out-spvasm: The .spv we generate from the files in the previous step.

Running cargo test generates files in the in-spvasm, out and out-spvasm directories. The CI check validates that files have not changed, and that the generated .spv files assembles and validates. If you are happy with generated changes as part of a PR, the updated snaphots should be included in the PR and be part of the review.

Future follow up work:

• For a subset of compute shaders with a unit like structure, actually run them and validate that tests/snapshots/in-spvasm and tests/snapshots/out-spvasm produce the same output.
  • I have this in the pipeline, will submit as a follow up once we're happy with this.
• Setup multiple output files for different transforms and passes?