VR: Add "absolute tracking" session option to UI

Exposes the absolute tracking/stage reference space functionality
introduced in rB36c0649d32aa.

viewport_vr_preview.py

@@ -332,6 +332,7 @@ class VIEW3D_PT_vr_session(Panel):
         layout.separator()
 
         layout.prop(session_settings, "use_positional_tracking")
+        layout.prop(session_settings, "use_absolute_tracking")
 
 
 class VIEW3D_PT_vr_info(bpy.types.Panel):