Audit what's supported or not in a bunch of WebXR demos

[nox]

• https://github.com/wizztjh/awesome-WebVR
• https://vr.mozilla.org/webvr-demos/
  • https://tutorialzine.com/2017/09/12-exciting-webvr-demos-and-experiments
  • https://webvr.info/samples/
  • https://aframe.io/
  • https://experiments.withgoogle.com/collection/webvr
  • https://vr.chromeexperiments.com/
  • https://webvr.rocks/firefox
  • https://developer.oculus.com/webvr/
  • https://www.awwwards.com/websites/webvr/
  • https://bball.surge.sh/golden-state-warriors
  • https://webvr.directory/

Check what's needed and not yet implemented in Servo (if anything) to run those WebXR demos.

[jdm]

Additionally, talk to other members of the XR team to gather representative content that they care about.

[jdm]

Within | https://vr.with.in/
Sketchfab | https://sketchfab.com/virtual-reality
Lifelique | https://www.lifeliqe.com/
Nearpod | https://nearpod.com/s/freevirtual-reality-C491,803
51VR | http://www.51hitech.com/
Here Be Dragons | https://www.dragons.org/
BioFlight | http://www.immersus.co/
VRchive | https://vrchive.com/
Konterball | https://konterball.com/

[Manishearth]

Splitting out WebAudio investigation into https://github.com/servo/servo/issues/21632

[asajeffrey]

The first blocker for sketchfab is navigator.vendor being undefined. Unfortunately fixing that (#23551) doesn't fix the viewer.