Map generation

[jtsymon]

Implement some form of map generator that produces cave/dungeon levels.

Maps are effectively arbitrary geometry represented by a graph of connected points. Should be possible to generate such a structure, or convert any known map structure to this format.

May need to change the structure somewhat to implement #3 if the textures are tied directly to the map elements rather than just a static background (especially if map elements can change/move: #8).
Might need to store the map with polygonal regions as hallways/rooms/whatever that have floor textures, rather than just storing the walls. This shouldn't affect collisions or shadow rendering at all, as those can still use the walls directly.