Fixes and improvements

-HUD now uses pre-generated vertices instead of generating this every
frame
-vr_ipd is now properly initialized on startup
-Rare issues with the crosshair has been fixed
-The aiming laser no longer needs the crosshair to be enabled to work
-vr_ovr_distortion has been updated so that 0 is display resolution, 1
is HMD native resolution, and 2 is 1/2 native, and 3 is 1/4 native
-various defaults have been updated
-r_gamma has been renamed (back) to vid_gamma