Added a disconnect packet; fixed-point positions

When a client disconnects from the server, everyone who's still
connected is notified of this so they can stop drawing that triangle.
Also, positions now have two binimal places for smoother paths near
cardinal directions.  This also means maps can be a quarter as big,
but they're still bigger than I think they'd ever need to be.  I may
revert this change at some point.  Finally, the client now draws its
own player above all the others, so you can't get hidden by other
folks driving on top of you.