* new structure: bridges
* add unwalkable cells around trees, greatly improves pathfinding
* greatly improve unit multi-selection accuracy
* opening maps in edit mode is now non-blocking (multi-threaded)
* tanks trails (v1) - will improve and will generalize to all vehicles
* better tree contrast in snow biome
* make ocean bottomless looking
* mission 7: require recapturing all stolen bases
* nuclear explosions and bombs don't affect airplanes anymore