[DRAFT] Implement XR_META_recommended_layer_resolution extension

[devloglogan]

Adds an extension wrapper implementation of XR_META_recommended_layer_resolution.

The extension can be enabled via project settings. When enabled, the extension will dynamically update the rendered image resolutions at a resolution of less or equal size to the initial swapchain size. I've primarily tested this just by spawning enough cube mushes in the demo project until FPS starts taking a hit, at which point the runtime will typically begin recommending lower resolutions as more cubes are added.

This PR relies on godotengine/godot#99407, so I'll keep it as a draft until those changes are merged.

[dsnopek]

I did some testing with this on Quest 3, and saw really great performance gains!

In my rendering benchmarking project, it gathers some metrics at 3 different "markers" in the scene, one of which usually drops down to 55fps (from the target 72fps which all the other markers reach). However, when running it with this extension enabled, it keeps a steady 72fps (at the cost of visibly losing resolution, but it's not terrible). I added some print statements and I can see it scaling the resolution down at that marker, and then scaling it back up at the next.

This is really cool! :-)

The code in the PR looks good to me as well.

[m4gr3d]

Looks good!