[XR] Foveated mode with optional eye tracking

[RaananW]

Implement (fixed) foveated rendering for XR scenes.

The general idea is to render the scene in low-quality for the entire view area, and only render the middle of the screen in high quality (as this is where the (fixed) eye is. For devices that support eye tracking (currently not available in WebXR), we will need to be able to set the "center point" which renders in higher quality.

This requires a small change in sub-camera rendering.

For further reading - https://developer.oculus.com/documentation/unreal/unreal-ffr/

[RaananW]

On a side-note, one of the new features (behind a flag) in the Oculus Quest browser is Foveated rendering for WebXR.
I assume performance will be better if done on the engine level, but it does work.

[cabanier]

FYI Oculus browser now support foveated rendering as defined in the WebXR spec.

[RaananW]

TODO - Update the XRWebGLLayer - https://immersive-web.github.io/webxr/#xrwebgllayer-interface and expose this in the XRSessionManager

[RaananW]

Now supported in the xr session manager