Material: Add allowOverride.

[Mugen87]

Fixed #30726.

Description

The PR introduces Material.allowOverride which controls whether the material can be overwritten by Scene.overrideMaterial or not. It is true by default.

With this new flag, the background textures and skyboxes are not affected by Scene.overrideMaterial, see #30726. If users render a scene with Scene.overrideMaterial e.g. to produce a normal pass, they must set the background to null. For a correct result, this was mandatory even before this PR but now the overall engine behavior is more consistent.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.02 78.26	336.08 78.29	+65 B +30 B
WebGPU	525.13 146.26	525.17 146.28	+41 B +23 B
WebGPU Nodes	524.59 146.16	524.63 146.18	+41 B +23 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.07 112.14	465.15 112.18	+87 B +41 B
WebGPU	597.91 162.53	597.97 162.56	+63 B +31 B
WebGPU Nodes	553.03 152.01	553.1 152.04	+63 B +31 B