Accept only vec3 (not vecN) for the cross built-in

[ErichDonGubler]

Connections

Resolves #6153.

Description

Previously, we only checked that a single type of vector of floats was being used as input for the cross built-in. Only vec3 should be permitted; so, narrow our checking of the vector type to 3 elements.

Testing

Tests have been added to ensure coverage of what we should and should not accept for the cross built-in.

Open questions:

• Do we need to permit more than just vec3 from language frontends other than WGSL? Nope!

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[ErichDonGubler]

@teoxoy: Thoughts on this question in the OP?

Do we need to permit more than just vec3 from language frontends other than WGSL?

[teoxoy]

I don't know, I'd have to check.

[teoxoy]

GLSL and SPIR-V say they must be vec3s as well.

[ErichDonGubler]

Ah, yep, all these sources state only accepting 3-component vectors:

• HLSL: https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-cross
• SPIR-V: https://registry.khronos.org/SPIR-V/specs/unified1/GLSL.std.450.html, Ctrl + F for cross.
• MSL: https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf, Table 6.11. Geometric functions in the Metal standard library, cross entry

There seems to be some divergence in the float types accepted on these platforms, but that's orthogonal to this PR. 😀

Thanks to @teoxoy for onboarding me onto the language spec. resources I cited above. 🍻