WebXRManager: Fix for hand controllers

[cabanier]

When the hand controllers go in and out of view, the browser is allowed to remove them from the input sources list.
The current implementation in the Oculus quest alway returns them, but that is not true going forward or in devices such as the Hololens.

On those devices, the right hand will be assigned the left controller which will cause it to deform.

This fix addresses that issues by looking up the correct controller instead of blindly using the index.

[mrdoob]

Thanks!

[hybridherbst]

@cabanier this sounds very similar to

• XR Controller events from multiple controllers in AR are sometimes mixed up / multitouch doesn't work #23187
• and the related PR WebXRManager: Fix input sources being sent to wrong controllers (touch AR) #23188

do you think it's the same issue? We also had to fix this for Hololens and also for phone-AR multitouch but it never got merged...

Another related issue where your input would be appreciated is

• XR: session events are incorrect or missing when using immersive-ar and multitouch #23574

[cabanier]

@hybridherbst yes, this is the same issue and I verified that the same problem existed on the Hololens.
We're using the same backend as HL which is likely why both devices have this problem.

There's still an issue where the pointing ray gets duplicated every time a hand goes out of view. I didn't get a chance to look into that yet.