PMREMGenerator: Don't pollute node material scope.

[Mugen87]

Related issue: -

Description

This PR introduces a new Weak Map in renderers/common/extras/PMREMGenerator.js to make sure the module does not store a uniforms dictionary in the object scope of node materials.

It also fixes the names of two module scope variables which should begin with a _.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.53 79.09	339.53 79.09	+0 B +0 B
WebGPU	491.01 136.43	491.02 136.44	+13 B +13 B
WebGPU Nodes	490.47 136.32	490.49 136.34	+13 B +10 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.42 112.15	465.42 112.15	+0 B +0 B
WebGPU	561.58 152.14	561.59 152.13	+13 B -5 B
WebGPU Nodes	516.97 141.71	516.98 141.7	+13 B -8 B