Explore OpenXR and OpenGL extensions for depth, vsync, and preemption priority about illixr

[mhuzai]

XR_KHR_composition_layer_depth allows the application to submit depth buffers along with rendered images. It has already been implemented in Monado, and we can use it once we upgrade to latest Monado.

XR_KHR_android_thread_settings allows the application to tell the runtime which of its threads are higher priority and which are normal/low priority, so that the runtime can schedule them accordingly. Unfortunately, it is only available on Android.

There is currently no extension for vsync. However, there is one in the works.

Given the above, no extra work is required on our behalf at the moment.