Auto merge of #27224 - asajeffrey:webxr-glwindow-spherical, r=Manishearth

Add support for webxr cubemap and spherical 360 rendering

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Part of rendering webxr content as 360 video

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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 back-end rendering

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