[naga msl-out] Fix truncation of pulled vertices

[andyleiserson]

The vertex pulling transform transforms vertex shaders:

to receive the vertex buffers, lengths, and vertex id as args, and bounds-check the vertex id and use the index into the vertex buffers to access attributes, rather than using Metal's [[stage-in]] assembled attribute data

It is possible for the vertices to require truncation upon loading. For example, the vertex buffer could define the texture coordinate as a vec3, but the vertex shader might not care about the z value and load the texture coordinate as a vec2. Before this change, the generated code would do this by casting, which seems to be allowed from float4 to float3, but not from float3 to float2. This PR changes the truncation to use a swizzle selector to select the appropriate number of components.

Fixes #7410

Testing
Adds more snapshot tests for the vertex pulling transform. I also tested that this fix does resolve the issue seen on the website linked from the original bugzilla issue.

Review note: there are four new snapshot tests, with suffixes -x1 through -x4, for loading vertices into a scalar, vec2, vec3, or vec4. The TOML files for all four are identical since they are describing the source vertex formats. The WGSL files are the same except for the type of variable that the vertex is loaded to.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[andyleiserson]

Since the swizzle selector tends to be buried at the end of a very long argument list, this comment helps to call out what's going on, but if it's preferred not to add anything extraneous to the generated shaders, I'd be fine removing it.

[teoxoy]

We sometimes add these sort of comments to help debugging - it should be fine.

[teoxoy]

The doc is now out of date.

[teoxoy]

Nice!