Added WebXR Spacewarp on top of Multiview

[KooIaIa]

Add WebXR SpaceWarp backend (requires multiview)

This PR adds a WebXR SpaceWarp render pass on top of the multiview pipeline. It creates a dedicated motion‑vector + depth pass and submits it through the WebXR Spacewarp extension.

Summary of Changes

• New WebXRSpaceWarp helper that manages the SpaceWarp render pass and target.
• WebXRManager owns the helper and exposes renderer.xr.spaceWarp.
• SpaceWarp is gated by renderer.spaceWarp and multiview support.

Re‑adds multiview fixes that were lost in an upstream update:
#25

Usage (very minimal)

Since it's doing a second render pass, it works by rendering meshes added to renderer.xr.spaceWarp.scene.
Three.js reuses the same geometry buffer so these meshes just need custom motion materials.

// Apps add motion‑vector meshes to the SpaceWarp scene.
const motionMesh = new THREE.Mesh(mesh.geometry, motionMaterial);
renderer.xr.spaceWarp.scene.add(motionMesh);
// The motion material must output clip‑space motion vectors per eye

For a full simple implementation with a complete motion shader look here:
https://krabbel.dev/p/social-crabs-enjoy

The A-Frame PR to use this is here:
aframevr/aframe#5795