The WebGPURenderer needs its backend initialized before any direct
`.render(scene, camera)` call. The commented-out `// renderer.init()`
said "Only use when using <DebugRenderer />", but the non-debug path
also calls direct render from the post-processing fallback
(post-processing.tsx:318), which throws "Renderer: .render() called
before the backend is initialized" on any browser that falls back to
the WebGL2 backend.
Switching the gl factory to an async function and awaiting init()
before returning is safe in both backends (init() is idempotent and
the async factory is a supported @react-three/fiber v9+ pattern), and
prevents the error in WebGL2 fallback.
