Updates related to OculusSDK

These commits do two major things:
* Allow adjusting of head-tracking prediction time through r_oculusrift_prediction. The value of this cvar is the number of milliseconds used for prediction. It still defaults to 40ms, but is capped at 75ms and any values less than or equal to zero will disable it entirely. The 75ms cap is entirely arbitrary, but values too large tend to cause LibOVR to return invalid position information. 
* Add support for the magnetic drift correction added in LibOVR 0.2.3. Drift correction can be controlled through r_oculusrift_driftcorrect. It defaults to a value of 1 (on) and setting it to 0 disables it.