WebGLRenderer: Add multi-bounce ambient occlusion

[mrdoob]

Related issue: #32067, #32073

Description

Adds a computeMultiBounceAO() function that accounts for light bouncing multiple times in occluded areas, based on surface albedo. This prevents overly dark AO on bright/colored surfaces while maintaining proper darkness in truly occluded areas.

• Uses the polynomial approximation from "Practical Real-Time Strategies for Accurate Indirect Occlusion"
• Applied only to diffuse indirect lighting

https://raw.githack.com/mrdoob/three.js/multibounce-ao-simple/examples/index.html

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.11 84.81	350.45 84.94	+341 B +128 B
WebGPU	604.06 169.4	604.06 169.4	+0 B +0 B
WebGPU Nodes	602.66 169.16	602.66 169.16	+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	482.11 119.69	+341 B +128 B
WebGPU	673.47 184.84	673.47 184.84	+0 B +0 B
WebGPU Nodes	615.47 168.09	615.47 168.09	+0 B +0 B