[DRAFT] WebXR: Add support for Space Warp

[dsnopek]

Depends on #97151

This implements WebXR Space Warp, which is supported on the Meta Quest browser.

In order to test, you'll need to:

1. Go to chrome://flags in the browser, search for “WebXR Space Warp” and change it from "Default" to "Enabled"
2. In your WebXR app, add both "layers" and "space-warp" to either WebXRInterface.required_features or WebXRInterface.optional_features
3. Check the FPS somehow (perhaps using the overlay from the OVR Metrics app?) and see that it is 45 fps (rather than the expected 90 fps -- unless you purposefully changed it to something else, in which case it should be half of whatever you set it to)
4. You can also enable "space warp debug mode" on the Quest using the adb commands shown in PR [DRAFT] Implement XR_FB_space_warp extension GodotVR/godot_openxr_vendors#222, which will show a transparent overlay of the motion vectors. Non-transparent objects that are moving should light up!

TODO:

• Set deltaPose for changes to the world origin in the same way as PR [DRAFT] Implement XR_FB_space_warp extension GodotVR/godot_openxr_vendors#222

Marking as a DRAFT until PR #97151 is merged