Support XR teleportation and more sophisticated inputs

[syntheticmagus]

Among other things, this will require implementing the more advanced features of XRReferenceSpace; this is what several TODO lines in the XRReferenceSpace implementation are referring to.

[bghgary]

@PolygonalSun Can you check if this is something you want to tackle?

[bghgary]

Closing for now. Reopen if we want to support this feature.