Blender Git Loki

Core XR Support [part 1]: Add OpenXR-SDK dependency and WITH_XR 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.
* I copied `presentation.cmake` and `xr_platform_defines.cmake` from the SDK. They contain logic that is not needed for us and prints at CMake generation. We could change that but figured it would also be helpful to keep the files equal to the SDK ones.
* For Windows a patch is needed to link the CRT in a compatible way.
* @LazyDodo already pushed the precompiled binaries for Windows.

All this is entirely untested on macOS.

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