Also, moved updateHeadPose so that the overlay and the main rendering use the same headPose.
Which should also be the same place the latencyMarker for the ovr_GetTrackingState is set.
Before this fix, a script could call into HMD.getHUDLookAtPosition2D() while the app was shutting down, which in turn would call
getHeadPose() on the currently active display plugin. This call could cause a crash within the openvr plugin, because the SDK was either shutdown, or in the process of shutting down on the main thread.
This fixes this by spliting the previous DisplayPlugin::getHeadPose(int) into two parts:
* updateHeadPose(int) which is only called once a frame and only by the main thread.
* getHeadPose() which is thread-safe and will return a cached copy of the hmd pose sampled by the last updateHeadPose.
* Call vr::VR_Shutdown() when ref-count goes to zero. The ref-count
is ncessary because the vr::IVRSystem poitner is shared between the
the openvr input and display plugins.
* OpenVR plugins options will display in the menu if vr::VR_IsHmdPresent() is true.
This is faster then initializing all of openvr and less likely to conflict with the
oculus display plugin.
