All DirectX management happens on Ghost level now, higher level code can
just assume everything is OpenGL (except of the upside-down drawing that
still needs to be done for DirectX). This is similar to how the
metal-layer is hidden outside of Ghost.
The Ghost-XR graphics binding for DirectX is responsible for managing
the DirectX compatibility now.
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
