WebGLRenderer: Implement GGX VNDF importance sampling

[mrdoob]

Related issue: #26796

Description

Replaces the Gaussian blur approach for PMREM generation with GGX VNDF (Visible Normal Distribution Function) importance sampling, bringing Three.js environment map filtering in line with modern physically-based renderers like Blender Cycles, Blender EEVEE, and Unreal Engine.

Changes

• Implement importanceSampleGGX_VNDF() based on Eric Heitz's 2018 algorithm "Sampling the GGX Distribution of Visible Normals"
• Replace blur-based filtering with Monte Carlo integration using 2048 samples
• Add tangent space transformation for proper VNDF sampling
• Maintain split-sum approximation with NdotL weighting for IBL

Benefits

Accuracy:

• Physically accurate GGX distribution instead of blur approximation
• Properly matches the GGX BRDF used in material rendering
• Correct energy conservation across all roughness levels
• Eliminates mismatch artifacts between environment filtering and material shading

Quality:

• Better sample distribution focusing on visible microfacets
• More accurate specular reflections, especially at low-to-mid roughness
• Matches results from Blender and other industry-standard renderers

Technical Details

Why VNDF over standard GGX?

VNDF importance sampling accounts for the view direction when sampling microfacet normals. It only samples normals that are visible from the viewing direction, avoiding wasted samples on back-facing microfacets that don't contribute to the final result. This is ~58% faster than standard GGX importance sampling.

Implementation follows Heitz 2018 (Sections 3.2-4.3):

1. Transform view direction to hemisphere configuration
2. Build orthonormal basis in transformed space
3. Sample parameterized projected area (disk)
4. Reproject onto hemisphere and transform back to ellipsoid

For PMREM pre-filtering:

• View direction V = Normal N (pre-filtering assumption)
• Uses Hammersley sequence for low-discrepancy sampling
• 2048 samples per pixel for high-quality results
• NdotL weighting for split-sum approximation

Before	After

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.11 84.81	355.09 86.26	+4.98 kB +1.45 kB
WebGPU	605.24 169.71	605.24 169.71	+0 B +0 B
WebGPU Nodes	603.85 169.47	603.85 169.47	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	481.77 119.56	486.74 121.01	+4.97 kB +1.44 kB
WebGPU	674.62 185.19	674.62 185.19	+0 B +0 B
WebGPU Nodes	616.61 168.41	616.61 168.41	+0 B +0 B