Allow keys to be held down and new keys to be pressed as well.
Each lane is position in front of or behind the others, need to add and remove the trucks to lanes so they appear in front of or behind each other.
Use lanes to determine where min and max of level are.  There is an off-by-1 between truck position and feature position, will probably need to be fixed when trucks are put into lanes.
Now trucks are drawn in front of and behind features and each other properly.
Draw a barrier instead of a reverse jump for barriers
Added boost things to run over, don't actually boost yet.
Boost works now though through hacky means.  Another TODO is to make features not land on top of each other.