Bug 315274 – WebGPU+WebXR on visionOS: setScissorRect unusable on XR projection layer

• Component: WebXR
• Status: RESOLVED FIXED

[Martin]

### Environment
- WebXR (immersive-vr) + WebGPU via XRGPUBinding

### Bug

Creating a projection layer yields:
  - colorTexture (attachment) extent: e.g. 1888×1792
  - subImage.viewport: e.g. 4338×3478 (larger than the attachment)

setViewport works fine with either size — including the oversized view extent.

setScissorRect cannot be used at all:
  - setScissorRect(0, 0, 1888, 1792)  // attachment size → renders only into top-left ~1/3 of the eye ❌
  - setScissorRect(0, 0, 4338, 3478)  // view size       → WebGPU validation error (scissor must fit attachment) ❌
  - no setScissorRect call             // default          → renders full eye ✅

Per WebGPU spec, scissor starts at the full attachment, so the attachment-sized call
should be a no-op equivalent to omitting it. On visionOS the explicit call appears to
be interpreted as a positional hint for the compositor's mapping of the layer texture
to the eye display.

### Impact

WebGPU engines that call setScissorRect as part of standard pass setup (PlayCanvas in this case)
render into a sub-rectangle of the eye on AVP. Engines/samples
that happen not to call it (Toji's vr-barebones, webgpu-metaballs) render correctly.
Current workaround: suppress setScissorRect entirely on the XR path.

### Repro
WebXR-Samples vr-barebones.html with one added setScissorRect call before the draw reproduces it.
https://github.com/immersive-web/webxr-samples

[Radar WebKit Bug Importer]

<rdar://problem/177664498>

[Mike Wyrzykowski]

 setScissorRect(0, 0, 1888, 1792)  // attachment size 

is correct based on the WebGPU specification as x, y, width, height are in pixels - I will fix this up

[Mike Wyrzykowski]

Pull request: https://github.com/WebKit/WebKit/pull/65631

[EWS]

Committed 314005@main (eda478cddb5d): <https://commits.webkit.org/314005@main>

Reviewed commits have been landed. Closing PR #65631 and removing active labels.