Hud is now affected by headtracking

Currently using a temporary method. It approximates the way the HUD is
supposed to move using 2D rotations, translations and scaling. It isn't
accurate, and unless adjusted, the crosshair won't be pointing at
exactly where you aim. I will convert this to a true 3D projection in
the future, then the crosshair will be accurate, and you won't have to
adjust other values whenever you change fov or hud_width.

Added CVARs:

vr_yawbias: Degree in radians. Determines where the "resting" position
is when vr_driftcorrection is enabled.

vr_hud_track_x: the speed multiplier for hud movement. You may need to
adjust this if you change your FOV or vr_hud_width

vr_hud_track_y: the speed multiplier for hud movement. You may need to
adjust this if you change your FOV or vr_hud_height