Function selectBackend

Select the rendering backend.

Precedence (highest to lowest):

1. Per-instance rendererOverride (URL ?renderer=webgl|webgpu).
2. VITE_LUXAR_USE_WEBGPU=1 / VITE_LUXAR_USE_WEBGPU_RENDERER=1 env vars.
3. VITE_LUXAR_USE_LEGACY_WEBGL=1 env var (no-op alias for the default).
4. Default: WebGL.