Depreciate r_mode

-Completely removed the numbered video mode system in favor of a system
based on explicit resolution sizes
-Removed r_mode, r_customwidth, and r_customheight
-Added r_width and r_height - specify width/height to use
-Changes to r_width and r_height only take effect after executing
vid_restart (or applying settings in the video configuration menu)
-Video options menu now sets r_width/r_height instead of r_mode.
-Unrecognized resolutions show up in the Video Option menu as "custom"
-Future versions will attempt to populate the available options in the
Video menu with only modes available