Switch headset on VR entry

[avaer]

Right now, Exokit Web has two types of "headset": the virtual one that is always there, and its binding to the real headset that the user is wearing.

The problem is when switching between the two: the number of eyes changes, as do the canvas metrics used.

Apps like CryptoVoxels aren't handling this gracefully; we might be able to shim in the right behavior with vrdisplaypresentchange or similar.

[avaer]

Note that this is not a problem when creating the xr-iframe from within VR, to load a VR site -- in that case the correct metrics are usually picked up during initiailization.