openvr-context: Adapt to OpenVR 1.8.19 scene transition changes
Scene applications do not get VREvent_SceneApplicationStateChanged when another scene app starts. They only get a VREvent_Quit. At this point they do not know whether they got this quit event because the user exited SteamVR, or because the user started another scene app.
xrdesktop for example wants to know this difference because if it runs in scene mode and the user starts another scene app, xrdesktop wants to transition to overlay mode and if the user quits steamvr, xrdesktop wants to quit.
However when scene apps get a VREvent_Quit they can look at the SceneApplicationState. If this state is "quitting" that seems to mean steamvr is trying to shut us down, in order to start another scene app. If this state is "running", that seems to mean steamvr is shutting down itself.