Examples: Update webgpu_postprocessing_sss using pre-pass

[sunag]

Related issue: #32295

Description

Add built-in support for SSS using pass context and update the example.

SSS Pre-Pass
SSS No Pre-Pass

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	358.3 86.96	358.3 86.96	+0 B +0 B
WebGPU	620.54 174.18	620.64 174.2	+103 B +25 B
WebGPU Nodes	619.14 173.92	619.25 173.95	+103 B +25 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.25 121.78	490.25 121.78	+0 B +0 B
WebGPU	691.41 189.78	691.52 189.81	+103 B +30 B
WebGPU Nodes	632.86 172.96	632.96 172.99	+103 B +29 B

[Mugen87]

For the future, do you think we could simplify this bit somehow? Maybe:

scenePass.contextNode = context( {
    sss: sssPass.getTextureNode().sample( screenUV ).r
} );

The instance of SSSNode has a reference to the directional main light so maybe you could hide some of this code in AnalyticLightNode.

[sunag]

Yes, I still have to think of a way to simplify this utilization.