Cleanup + add vr_force_resolution

-Cleaned up the HMD->VR interface a bit
-Added vr_force_resolution that forces display resolution to the
expected HMD resolution
Shuffle options menus to be higher on the screen
OVR Backend now properly adjusts render target resolution

Previously, backend always rendered at native resolution for device. Now
if you turn down (or up) resolution it will scale appropriately.