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!

[devloglogan]

Opening this since godotengine/godot#99407 was merged :)

[dsnopek]

@devloglogan Could you rebase on master? You'll need to get rid of your changes to extension_api.json to clear up the conflicts. And I suspect this will pass tests due to other changes on master too

[devloglogan]

@dsnopek rebased :)

[dsnopek]

Thanks!

I just tested the latest version of this on Meta Quest 3 with my rendering benchmarking project, and it's working great with Godot 4.4-beta1! With recommended layer resolution turned on, I'm still seeing a steady 72 FPS (at the cost of visibly lower resolution) when going to the marker that usually has 55-60 FPS.