Added hooks for maintaining separate Rift/non-Rift field of view (see below)
CVAR rend-camera-fov always represents the current field of view, whether in Rift mode or not (so culling can work correctly).
CVAR rend-vr-rift-fovx represents stored/current fov for Rift mode.
CVAR rend-vr-nonrift-fovx represents stored/current fov for other modes.
Changing VR::mode, camera-fov, rift-fov, or nonrift-fov each can cleverly update the other fov values, when appropriate.
Add command for loading VR parameters (IPD & aspect) from Oculus Rift SDK
Add CVAR for whether to automatically load VR parameters from SDK
But don't load field of view, because result does not seem correct.