WebXRManager: Fix caching of baseReferenceFrame and teleport example.

[richardanaya]

Fixed #24033

Description

The teleporter example was broken in a few ways.

• the example wasn't creating a transform from the original baseReferenceFrame acquired from the WebXR manager, leading to really strange offsets after the initial teleport. The first teleport would look correct, but then you'd be modifying a modified space.
• the baseReferenceFrame wasn't nulled out on sub-sequent entering of vr sessions, only allowing you to enter into the example with VR once
• the example needs to watch for changes in the session to re-grab baseReferenceFrame again

Video of it working:

a93b746eb1f14bf512196d6628573812.mp4

[richardanaya]

@arodic thanks for the review :) made that change on removing event handlers as you suggested, I left a comment on why I need to cache the session.

[arodic]

Excellent! I can confirm that this fix works as intended (tested in Oculus Quest 2). I can also confirm that without this fix, the example is broken if multiple sessions are attempted.

[LeviPesin]

@richardanaya Can you please update the PR with a sessionstart listener instead of that check?

[mrdoob]

@LeviPesin Where would that listener go?

[LeviPesin]

I think https://github.com/mrdoob/three.js/pull/24058/files#diff-28e2f2792c9f311cb52ce985452165df772dcfde81e80f00cd699426ede935c6R107-R116 should be reverted and to the init() function line renderer.xr.addEventListener( 'sessionstart', () => { baseReferenceSpace = renderer.xr.getReferenceSpace(); } ) should be added.

[richardanaya]

@LeviPesin fixed and re-tested according to your suggestion, it's much cleaner :) thank you

[mrdoob]

Thanks!