WebGLRenderer/WebGPURenderer: Simplify multi-scattering compensation for direct lighting

[mrdoob]

Related issue: #32072 #32089

Description

This PR simplifies the multi-scattering compensation in the GGX BRDF for direct lighting, aligning both WebGL and WebGPU implementations with the standard Fdez-Agüera approximation.

Previously, BRDF_GGX_Multiscatter used a complex dual-DFG approach that computed separate energy terms for both view and light directions. This has been simplified to use a single DFG lookup based on the view direction only, matching the approach already used in computeMultiscattering for indirect lighting.

The simplified implementation:

• Uses one DFG lookup instead of two (better performance)
• Matches the standard Fdez-Agüera formula from the paper "Multiple-Scattering Microfacet Model for Real-Time Image Based Lighting"
• Provides consistent multi-scattering behavior between direct and indirect lighting
• Maintains proper energy conservation for rough metallic surfaces

Changes:

• src/renderers/shaders/ShaderChunk/lights_physical_pars_fragment.glsl.js - Simplified BRDF_GGX_Multiscatter
• src/nodes/functions/BSDF/BRDF_GGX_Multiscatter.js - Updated TSL implementation to match

Reference: http://www.jcgt.org/published/0008/01/03/

For comparison:
https://raw.githack.com/mrdoob/three.js/multiscattering/examples/index.html

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.11 84.81	349.63 84.67	-480 B -134 B
WebGPU	604.15 169.43	604 169.37	-151 B -57 B
WebGPU Nodes	602.75 169.19	602.6 169.13	-151 B -60 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	481.77 119.56	481.29 119.42	-480 B -139 B
WebGPU	673.57 184.91	673.42 184.85	-151 B -54 B
WebGPU Nodes	615.56 168.12	615.56 168.12	+0 B +0 B

[mrdoob]

Lets keep Fdez-Agüera for IBL and the bidirectional formulation for discrete lights.