Blender Git Loki

XR: Add runtime window area for XR events

This adds an offscreen View3D window area for the VR view in order to
execute XR events/operators in the proper context. The area is created
as runtime data before XR events are dispatched and set as the active
area during XR event handling.

Since the area is runtime-only, it will not be saved in files and since
the area is offscreen, it will not interfere with regular window areas.
The area is removed with the rest of the XR runtime data on exit, file
read, or when stopping the VR session.

Note: This also adds internal types (EVT_DATA_XR, EVT_XR_ACTION) and
structs (wmXrActionData) for XR events.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D12472