[WebGPU-XR][Phase 3] WebXR camera projection matrix / NDC handling for WebGPU

[RaananW]

Part of #18635. Depends on Phase 2.

Phase 3 — WebXR camera projection matrix / NDC handling for WebGPU

Ensure camera math is correct for WebGPU XR views.

Scope

• WebGPU XR XRView.projectionMatrix uses a [0, 1] clip-space depth range, while the WebGPU engine already sets isNDCHalfZRange = true. Confirm WebXRCamera consumes the view projection matrices without double-converting the depth range.
• Validate interplay with reverse-depth buffer settings.
• Ensure viewport / scissor per sub-image is applied correctly.

Acceptance

• Depth and projection are visually correct in-headset on WebGPU (no clipping/precision artifacts); WebGL2 XR unchanged.