Blender Git Loki

Core XR Support [part 1]: Add OpenXR-SDK dependency and WITH_OPENXR build option

Some points on the OpenXR-SDK dependency:
* The repository is located at https://github.com/KhronosGroup/OpenXR-SDK (Apache 2).
* We use the OpenXR loader lib from it, the headers, and some CMake utilities.
* It contains a bunch of generated files, for which the sources are in a separate repository.
* To use the injected OpenXR API-layers from the SDK (e.g. API validation layers), the SDK needs to be compiled from this other repository.
* We could use that other repo by default, but I'd rather go with the simpler solution and allow people to opt in if they want advanced dev features.
* For Windows a patch is needed to link the CRT in a compatible way.

All this is entirely untested on macOS.

Maniphest Tasks: T71365

Differential Revision: https://developer.blender.org/D6188