WebGPURenderer: Implement GGX VNDF importance sampling for PMREM (#32134)

* WebGPURenderer: Implement GGX VNDF importance sampling for PMREM

Port the GGX VNDF (Visible Normal Distribution Function) importance
sampling implementation from WebGLRenderer's PMREM to WebGPURenderer's
TSL-based PMREM generator.

This implementation provides more accurate environment map prefiltering
by using Monte Carlo integration with VNDF importance sampling to
represent the GGX BRDF for physically-based rendering.

Changes to PMREMUtils.js:
- Added GGX VNDF sampling helper functions:
  - radicalInverse_VdC: Van der Corput radical inverse
  - hammersley: Hammersley sequence for quasi-Monte Carlo sampling
  - importanceSampleGGX_VNDF: GGX VNDF importance sampling (Heitz 2018)
  - ggxConvolution: Main convolution function using VNDF sampling
- Optimized shader code by removing unnecessary .toVar() calls

Changes to PMREMGenerator.js:
- Added GGX_SAMPLES constant (1024 samples, optimized for performance)
- Removed _axisDirections (no longer needed with GGX filtering)
- Added _ggxMaterial property
- Replaced blur-based _applyPMREM with GGX filtering
- Added _applyGGXFilter method for incremental roughness filtering
- Added _getGGXShader function to create GGX material
- Updated documentation to reflect GGX VNDF usage

Technical notes:
- Uses texture() instead of texture(null) for EmptyTexture default
- Helper functions don't use setLayout (TSL pattern for nested Fn)
- Implements incremental roughness filtering to avoid over-blurring
- Applies blur strength mapping (0.05 + roughness * 0.95) for quality
- Performs two-pass rendering: pingPong target then back to cubeUV
- Reduced sample count to 1024 (vs WebGL's 2048) for better performance

* Clean up.

* Updated screenshots.

* WebGPURenderer: Remove unnecessary .toVar() from s variable

Performance optimization: removed .toVar() from the s variable in
importanceSampleGGX_VNDF as it is never reassigned and only used
for reading on the next line.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* Remove `.toVar()`

* Revert "Remove `.toVar()`"

This reverts commit 3bfdc12.

* update screenshots

* update screenshots

* Increase GGX samples from 1024 to 2048

* Reduce GGX samples from 2048 to 1024

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Co-authored-by: Claude <[email protected]>
Co-authored-by: sunag <[email protected]>

Author: 3 people