Unified terrain area and terrain mod handling

The code and format for handling terrain areas and terrain mods are different. This is unfortunate since they basically do the same thing: alter a 2d terrain bitmap. In the terrain area case a layer is altered, and in the mod case a height map is altered. The code for these two functions could be combined.

Furthermore, the atlas protocol format differs. While the mods can use any arbitrary WFMath shape, the areas can only use polygons. It would be good if the format for the shapes would be the same, and if indeed the whole format could be standardized and documented.