UE-30441 - OSVR continues to render on HMD when not in VR Preview or Stereo On

[JeroMiya]

1. Set up OSVR
2. Start OSVR_Server
3. Run Ue4Editor.exe QAGAME Tm­VRSMOKE
4. Play in VR­Preview
5. Exit VR­Preview
6. Look through the HMD
   Result: Last frame is still rendering on the HMD, this could easily cause burn in if the editor was left up for a period of time.

notes:

If you exit stereoscopic rendering without closing the UE4 instance, the OSVR continues to render the last passed frame. This occurs in ­game if you enter the 'Stereo off' command, and in editor if you simply end VR­Preview, but don't close the editor.

My notes: I think we've covered what we can for shutting down the HMD when the user exits the VR Preview, but we haven't handled when the user enables or disables stereo while the game is running.

[JeroMiya]

Fixed part of this with #107

Specifically, if you type stereo on or stereo off while playing, it will now start or stop the custom present and start/shutdown render manager.