Implement partially decoupled aiming mode

-Implemented as r_oculusrift_aimmode 5
-Similar to vr_moveaim_mode 3 for TF2
-Mouse can free aim within a deadzone. Moving to the edges will turn the
view.
-Viewing is decoupled, but will move the point of aim when it reaches
the edge of the dead zone (for yaw).
-r_oculusrift_aim_deadzone can adjust the angular area of the deadzone,
and is limited to range 0-70 inclusive. Setting values outside this
range sets it to the closest edge of the range.
Cleanup

Switched a few places to using CLAMP() macro.
Updated r_oculusrift_crosshair

-Crosshair is scales so it is the equivalent of 3 pixels wide for point,
2 pixels wide for line
-Point is now a rounded dot
-Added mode 3, which is a point projected (roughly) to infinity