ShadowNode: Ensure background does not influence shadow map.

[Mugen87]

Fixed #30111.

Description

The PR makes sure the scene background does not affect the shadow map anymore.

@sunag We might want to rename PostProcessingUtils to RendererUtils maybe^^?

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.4 79.05	339.4 79.05	+0 B +0 B
WebGPU	486.67 135.01	486.76 135.02	+95 B +15 B
WebGPU Nodes	486.13 134.91	486.23 134.94	+95 B +32 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.3 112.11	465.3 112.11	+0 B +0 B
WebGPU	555.76 150.47	557.09 150.77	+1.32 kB +297 B
WebGPU Nodes	511.83 140.23	513.15 140.53	+1.32 kB +295 B

[sunag]

@sunag We might want to rename PostProcessingUtils to RendererUtils maybe^^?

I'm happy we're so aligned, as I was considering this as well :)

[Mugen87]

The PR breaks the motion blur demo. Investigating...

[Mugen87]

There was actually a bug in resetRendererAndSceneState() (no reset for renderer properties happened). I have refactored the module slightly so this oversight does not happen anymore.

[Mugen87]

@sunag With this PR, the shadows in webgpu_shadowmap_opacity look a bit darker. That happens because the scene's background does not influence the shadow map anymore (verified this by toggle the scene reset). I think the shadow color is now more correct.