Document the sample_mask WGSL builtin

[mvaligursky]

Important

Do not merge until Engine 2.22 is released. The sample_mask builtin support ships in Engine 2.22 (playcanvas/engine#9222, merged to main). Merge once 2.22 is out.

Documents the WGSL sample_mask builtin on the WGSL Vertex and Fragment Shaders page, alongside the existing sampleIndex / fragDepth builtins (WebGPU only).

Changes:

• FragmentInput builtins: sampleMask: @builtin(sample_mask) (the fragment's MSAA coverage mask), also available in global scope as pcSampleMask.
• Fragment Shader Outputs: sampleMask: @builtin(sample_mask) next to fragDepth, with a short explanation of the semantics (the written mask is AND-ed with the coverage mask by the GPU, allowing per-sample discard - the basis of custom alpha-to-coverage dither patterns) and a minimal example.
• Japanese translation updated to match.