Add renderer backend and reversedDepthBuffer properties, pass multiview option to WebGPURenderer

[vincentfretin]

Fixes #5749

New renderer properties

• multiviewStereo is now also passed as the multiview option used by WebGPURenderer (WebGLRenderer keeps reading multiviewStereo, each renderer ignores the other's key).
• New reversedDepthBuffer property, passed as-is to both WebGLRenderer and WebGPURenderer.
• New backend property (webgpu or webgl): backend: webgl maps to the forceWebGL option of WebGPURenderer. When unset, nothing is passed so the WebGPU build keeps its current behavior (try WebGPU, fall back to WebGL2). Ignored by WebGLRenderer.

Wait for async renderer init before starting the render loop

WebGPURenderer initializes its backend asynchronously via init(). Previously renderStarted was set and renderstart emitted while the backend was still initializing (three.js setAnimationLoop recovers by awaiting init internally, but anything reacting to renderstart calling e.g. renderer.render() or compileAsync() would throw .render() called before the backend is initialized). The render loop kickoff now awaits renderer.init() when it exists; WebGLRenderer has no init method and keeps the fully synchronous path.

Tests

The renderer attribute parsing is extracted from setupRenderer() into getRendererConfig() so it is unit-testable without creating a WebGL context (setupRenderer is stubbed on CI). Added tests for multiviewStereo/multiview, reversedDepthBuffer, backend and the async init behavior. There were no existing tests covering multiviewStereo.

Also adds a code note that setPoseTarget (A-Frame specific super-three patch to WebXRManager) will need an equivalent in the XRManager used by WebGPURenderer when WebXR support is added.

Verified at runtime with both builds (npm run dev and npm run start:webgpu) driving headless Chrome: options reach the renderer (backend.isWebGLBackend, reversedDepthBuffer, xr._useMultiviewIfPossible) and renderer.initialized is now true when renderstart fires. Actual multiview rendering in VR still needs a headset test with examples/performance/multiview-extension.

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