Implement reset key in OVR; add IQuatTracker for trackers that export quaternion rotation

* Implement "reset camera" key for Oculus OVR
* Add IQuatTracker interface for head trackers that are able to export authoritive rotation data as quaternions, and implement it on the Oculus tracker. This prevents issues when looking straight down. For those trackers, we cannot adjust rotation; however, this is rarely wanted when using VR trackers because incorrect rotation makes people sick anyways, so we just trust what the tracker tells us.