Blender Git Loki

VR: Fix OpenXR state freeze on Oculus after taking off HMD

With the Oculus runtime, the VR session would freeze when taking off the HMD
and putting it back on. This was caused by the deletion of graphics resources
too early in the OpenXR state machine, at least for Oculus.
The resources will now only be freed once the session is actually destroyed.

Also fixes an issue where it wasn't possible to stop the session via the UI
when the HMD was taken off.

Reviewed By: Julian Eisel

Differential Revision: https://developer.blender.org/D7635