* 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.
