Blender Git Loki

Proper creation & destruction of OpenXR instances

Adds wm_xr.c for an XR management API and creates/destroys the OpenXR
instance through this. This is as planned in my proposal, to lock the
OpenXR calls behind an abstraction.
XR data will be stored in a (non-public) wmXRContext struct within the
window-manager.

For now, creates the OpenXR instance on startup. I think it's better to
lazy setup this, as in, only creating the instance once the user starts
the first XR session. Just to avoid costs for something that may not be
used (the OpenXR loader we use will try loading and parsing the system's
OpenXR active_runtime.json on instance creation). That's for later when
I introduce session management though.

Also added a context getter for the xr-context, which is unused but may
be handy later.