WebGLRenderer: Add more energy conservation for physically-based materials

[mrdoob]

Description

This PR implements proper energy conservation in the physical lighting model by ensuring that diffuse and specular reflections correctly account for the Fresnel effect.

Previously, the diffuse term did not account for energy taken by specular reflection, causing materials to appear over-bright, especially at grazing angles. This violated the fundamental principle that reflected energy cannot exceed incident energy.

The implementation modifies RE_Direct_Physical and RE_IndirectDiffuse_Physical to attenuate the diffuse contribution by (1.0 - F), where F is the Fresnel term. This ensures total reflected energy (diffuse + specular) does not exceed incident energy, and diffuse contribution properly decreases as specular increases, particularly at grazing angles.

The changes result in more physically accurate rendering, especially for metallic and glossy surfaces. Materials no longer appear over-bright at grazing angles, metals show reduced diffuse at high metalness values, and Fresnel response is more accurate across all viewing angles.

Performance impact is minimal - the implementation reuses the existing Fresnel calculation with one additional multiply per light source.

For comparison:
https://raw.githack.com/mrdoob/three.js/webgl-pbr/examples/index.html

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.11 84.81	350.58 84.88	+474 B +73 B
WebGPU	604.15 169.43	604.15 169.43	+0 B +0 B
WebGPU Nodes	602.76 169.19	602.76 169.19	+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	482.24 119.64	+474 B +76 B
WebGPU	673.57 184.88	673.57 184.88	+0 B +0 B
WebGPU Nodes	615.56 168.12	615.56 168.12	+0 B +0 B

[mrdoob]

Not sure about this one.