Only emit written color outputs in WGSL FragmentOutput struct

[slimbuck]

Summary

The WGSL shader processor previously included all color output members in the
FragmentOutput struct based on numRenderTargets, regardless of whether the
shader actually writes to them. This can cause WebGPU validation errors when a
fragment shader declares an output location it never assigns to.

This change updates generateFragmentOutputStruct to scan the shader source for
actual assignments (e.g. .color = , .color1 = ) and only emit @location
members for outputs that are written, matching the existing pattern already used
for fragDepth.

Changes

• WebgpuShaderProcessorWGSL.generateFragmentOutputStruct now filters color
  outputs by checking if the shader source contains an assignment to each color
  member before including it in the struct

[Copilot]

Pull request overview

This PR fixes WebGPU validation errors that occur when fragment shaders declare color output locations they never assign to. The WGSL shader processor is updated to conditionally emit color outputs in the FragmentOutput struct based on whether the shader actually writes to them, matching the existing pattern used for fragDepth.

Changes:

• Modified generateFragmentOutputStruct to scan shader source for color output assignments before including them in the struct
• Uses regex pattern matching to detect .color = , .color1 = , etc. assignments in the shader source

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