various fixes and improvements

- fixed tank loading to update the 'control' property
- CANVAS.setup and CANVAS.clear now requires 2 arguments, the canvas and
the context
- tried drawImage for projectiles, cpu time soared, so it's been
commented-out temporarily
- added new assets, dirt-and-grass tiles (for random terrain generator -
to be implemented in the future)
- added heavy-class tank, Heavy
- improved pathfinder, now adjusts grid closed paths based on tank
collision size
- improved