Fixes for XR animation frame loop

[lojjic]

Fixes:

• Issue #115: rework xrSession.requestAnimationFrame to use a single device frame handler and a list of callbacks, to match the spec algorithm and prevent multiple vrDisplay.submitFrame calls per frame
• Issue #114: Instantiate new XRFrame on each frame and each input source event dispatch
• Add active and animationFrame booleans to XRFrame, toggle them appropriately, and check them during pose population per spec

Notes for Review:

• Given how central the frame loop is, please be very critical in reviewing this logic.
• My testing has been only on Oculus Quest, and only via my own framework code [edit: and the webxr-samples repo]. I would appreciate help testing on other devices, and if there are some other good testcases out there I should be aware of to run this against I'd love to know about them.
• The test-xr-session.js test file is busted so I haven't added anything there to test these changes, but I'd feel much better if I could, so I may try to at least get that one into a runnable state so I can add to it.
• I removed the suspendedCallback in favor of the callbacks array, and I think the logic around suspension (pausing/restarting the device frame loop) is correct, but I don't know how to actually test the suspension. Tips welcome.

[lojjic]

No worries! As conflicts go this is an easy one to unwrap, but thank you for the resolution tips.

Your other comments all make perfect sense, I'll work through them shortly.

[lojjic]

Conflicts are resolved (via rebase) and all the fixes from that first round of code review have been made.

I've now tested this with the webxr-samples repo without issue, but still welcome additional testcase sources. As well as hints around how to test the session suspension logic.

[toji]

Looks great! Thank you so much!