Blender Git Loki

Build System: Add OpenXR-SDK dependency and WITH_XR_OPENXR build option

The OpenXR-SDK contains utilities for using the OpenXR standard
(https://www.khronos.org/openxr/). Namely C-headers and a so called
"loader" to manage runtime linking to OpenXR platforms ("runtimes")
installed on the user's system.

The WITH_XR_OPENXR build option is disabled by default for now, as there
is no code using it yet. On macOS it will remain disabled for now, it's
untested and there's no OpenXR runtime in sight for it.

Some points on the OpenXR-SDK dependency:
* The repository is located at
https://github.com/KhronosGroup/OpenXR-SDK (Apache 2).
* Notes on updating the dependency:
https://wiki.blender.org/wiki/Source/OpenXR_SDK_Dependency
* It contains a bunch of generated files, for which the sources are in a
separate repository
(https://github.com/KhronosGroup/OpenXR-SDK-Source).
* 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.
* We currently use the OpenXR loader lib from it and the headers.
* 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.

The extra "XR_" prefix in the build option is to avoid mix-ups of OpenXR
with OpenEXR.

Most of this comes from the 2019 GSoC project, "Core Support of Virtual
Reality Headsets through OpenXR"
(https://wiki.blender.org/wiki/User:Severin/GSoC-2019/).

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

Reviewed by: Campbell Barton, Sergey Sharybin, Bastien Montagne, Ray
Molenkamp