WebGLRenderer: Unify cubemaps and cubeuvmaps into WebGLEnvironments

[mrdoob]

Consolidates WebGLCubeMaps and WebGLCubeUVMaps into a single WebGLEnvironments class with a cleaner API.

Before:

const envMap = ( material.isMeshStandardMaterial ? cubeuvmaps : cubemaps ).get( texture );

After:

const envMap = environments.get( texture, usePMREM );

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.82 84.56	355.77 84.57	-54 B +14 B
WebGPU	621.5 172.57	621.5 172.57	+0 B +0 B
WebGPU Nodes	620.1 172.33	620.1 172.33	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.06 119.45	488.01 119.48	-48 B +30 B
WebGPU	691.97 187.85	691.97 187.85	+0 B +0 B
WebGPU Nodes	641.77 175.02	641.77 175.02	+0 B +0 B