Blender Git Loki

Set up OpenXR views and spaces for drawing

Using a dummy identity pose at {0, 0, 0} to start with. This should
later use the current viewport position & orientation I guess.

Also adds function to set a draw callback. There's quite some OpenXR
related stuff to be done before and after drawing anyting, as well as
before and after drawing each view (eye). Quite some info would have to
be exposed to WM to let it manage drawing. So I think using a callback
called from GHOST_Xr to draw each eye instead is a good way to go.

VR session currently crashes after opening. Seems to be related to
blocking frame wait call. I'm not too worried about that though, might
disappear once OpenXR frame sync functions get proper timing info
passed.