Get WebXR minimally working again in Godot 4

[dsnopek]

~This is a draft PR, because it includes the changes from PR #65800 which is necessary for this to work.~

Not a draft anymore!

This includes the changes from PR #55869, which got merged into 3.x only, since it was impossible to port to Godot 4 without WebGL support (which we now have, yay!)

With the PR, WebXR will render fine on mono devices (like smartphone AR), but with stereo devices it will only render the left eye, because the 'opengl3' driver doesn't support multiview yet.

[dsnopek]

I rebased this on PR #65800, which is the latest attempt to fix the rendering issues with XR on the 'opengl3' driver. I think this one is the one! ;-)

For whatever reason, this PR is no longer working on the WebXR emulator (I suspect it's something related to PR #65727) but it still works fine on an actual device.

Once PR #65800 (or its successor :-)) gets merged, this one will still be ready to go in: it accomplishes what aims to do (get WeXR minimally working) and I can fix the issue with the WebXR emulator in a follow-up PR.

[dsnopek]

PR #65800 was merged (yay!) so I just rebased and re-tested this: it's still working just as well as my last comment!

This one is ready to go :-)

[akien-mga]

Thanks!