Draw-manager mutex has to be set before activating OpenGL/GPU context.
Otherwise, parallel jobs (like preview rendering) may try to activate
the context from another thread.
Also: Use WM wrappers for activating/releasing OpenGL context, which
have an additional assert check.
Suggest to backport this for 2.83.1.
The offsets are applied after toggling positional tracking off, so that
the view does not jump at that moment. But when changing the base pose,
keeping that offset doesn't make sense. Especially with landmarks, which
are supposed to give precise positions/rotations to jump to. For that
part the VR Scene Inspection Add-on will need a little adjustment
though.
Also exposes an explicit function to the Python API to reset the
offsets, to be used by the Add-on.
This is mostly untested since I don't have access to an HMD currently.
Splits up wm_xr.c into multiple files in their own folder:
source/blender/windowmanager/xr. So this matches how the message bus and
gizmo code have their own folder and files.
This allows better structuring and should make the code scale better.
I rather do this early on than to wait until we end up with a single,
huge file.
Also improves a bit how data is prepared and updated for drawing.
