Blender Git Loki

Ghost: Ghost-XR API to abstract away and access OpenXR functionality

Extends Ghost to include an abstraction for OpenXR, which I refer to as
Ghost-XR. Such an API is the base for the following commit, which introduces VR
support to Blender.

Main features:
* Simple and high-level interface for Blender specific code to call.
* Extensible for muliple graphics backends, currently OpenGL and a DirectX
compatibility layer are supported.
* Carefully designed error handling strategy allowing Blender to handle errors
gracefully and with useful error messages.
* OpenXR extension and API-layer management.
* OpenXR session management.
* Basic OpenXR event management.
* Debug utilities for Ghost-XR and OpenXR

For more information on this API, check
https://wiki.blender.org/wiki/Source/Interface/XR.

Reviewed by: Brecht Van Lommel

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