Overhead map additions

Fixed some issues related to overhead map (issue #85):
- No need to duplicate entire terrain - use of layering should solve that. This would enable easier replacement of terrain rather than require it to appear twice
- Changed the way the cart is followed, after some testing it seems that when the cart is turned over it disappears from the map until the game returns it to the right position. This is undesirable. Change also allows to perform easier manipulations to the shape if needed.