* map [+desert] added crashed Supply UFO
* map [+desert] added Bomber UFO
* simplified tile descriptors in desert.ump
* map [+desert] added crashed Harvester UFO to large assembly also
* map [+desert] added crashed Corrupter UFO
* map [+africa] added crashed Corrupter UFO
* raised MAX_MAPDEFS
* additional mapdefs for +africa theme