WebGPURenderer: Allow passing a custom backend

[pepperoni505]

Description

Renderer already delegates all GPU work to a Backend and accepts one in its constructor, but WebGPURenderer always constructs its own - WebGPU, or WebGL when forceWebGL is set. There's no way to run the renderer against a backend that isn't one of those two, which rules out things like a headless/context-free backend or one that records the command stream instead of issuing GPU calls.

This adds a backend option to WebGPURenderer. If you pass one it's used directly. If you don't, the existing WebGPU/WebGL selection is untouched, so nothing changes for current code.

const renderer = new WebGPURenderer( { backend: myBackend } );

For this to be usable outside the library, Backend and WGSLNodeBuilder are now exported from three/webgpu (GLSLNodeBuilder already was). I dropped the @private tag on Backend and documented it as a low-level, experimental extension point whose interface may still change, since it seems the API around it is still moving.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.43 86.79	365.43 86.79	+0 B +0 B
WebGPU	660.98 182.51	661.05 182.53	+71 B +22 B
WebGPU Nodes	659.1 182.23	659.1 182.23	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	498.3 121.53	498.3 121.53	+0 B +0 B
WebGPU	732.54 197.36	732.57 197.37	+35 B +13 B
WebGPU Nodes	681.76 184.75	681.76 184.75	+0 B +0 B