Blender Git Loki

XR: Support for Varjo OpenXR extensions

This adds support for two Varjo specific OpenXR vendor extensions:
1) XR_VARJO_QUAD_VIEWS
2) XR_VARJO_FOVEATED_RENDERING

Together these enable human eye resolution rendering on supported
devices (currently mainly Varjo XR-3 and VR-3).

In addition, there's a detection for Varjo OpenXR runtime.

This has been tested on real Varjo XR-3 hardware and Varjo Simulator
and confirmed to function correctly. Foveation works, and the views are
rendered correctly for all the four views.

Reviewed By: Peter Kim, Julian Eisel

Differential Revision: https://developer.blender.org/D12229