Auto merge of #26927 - asajeffrey:webxr-layer-management, r=Manishearth

Support for webxr layer management

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This adds layer management to servo's webxr implementation, and removes all the special-cased code for openxr surface creation and opaque framebuffers.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because it's all back-end plumbing

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