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.
