Needed for DirectX-only OpenXR runtimes (e.g. Windows Mixed Reality).
Adds a minimal DirectX 11 Ghost context, plus some shared DirectX-OpenGL resource interface using the NV_DX_interop2 WGL extension.
I know that the current implementation fails on some systems, which is something I'll have to fix at some point. Don't know if this is a showstopper though, I can fix that in master too. For now this isn't going to be used by many people anyway. Recently I also learned that OSVR uses the same extension, see https://github.com/sensics/OSVR-RenderManager/blob/master/osvr/RenderKit/RenderManagerD3DOpenGL.cpp. Their implementation may be useful to fix the issue, according to a OSVR dev, it works quite reliably for them.
Note: Didn't actually test just this patch on Windows yet.