Some bug fixes + features

New features:
* Can set IPD (in mm) in the console through r_oculusrift_ipd.
  * Defaults to LibOVR provided IPD value
  * Clamps values to 0mm to 100mm inclusive
  * Other values set r_oculusrift_ipd to LibOVR provided value so you can reset
* Setting r_oculusrift_prediction prints the prediction time on the console

Changes:
* Magnetic drift correction is disabled by default
* Chromatic aberration is enabled by default
* hmd_settings_t was moved into oculus_sdk.c
* Some rendundant calculations were removed from R_InitHMDRenderer()
* Resources for magnetic drift support are created/destroyed when it is enabled/disabled

Fixes:
* V_PolyBlend() no longer breaks the HUD when Rift support is enabled
* When you die the Camera no longer rotates around yaw axis when Rift support enabled
* The HUD no longer gets stuck rotated around the yaw axis when you reset the Rift home position
* Magnetic drift support now properly re-enables after resetting the Rift home position