Streamlined map generation

Made map::generate use absolute overmap terrain coordinates. It is supposed to generate all submaps that belong to one overmap terrain tile (that are 2x2 submaps) at once.
This also makes calling it from map::loadn easier. There is no overmap pointer passed to map::generate anymore. This moves the coordinate transformations to map.cpp only.

Also made map::saven take absolute world coordinates, instead of relative and the overmap pointer.

There is still a wrapper for the old map::generate because editmap uses it.
change mapgen json to support bigger than 1x1 maps

Gives overmap terrain a flags field

overmap terrain can have the GEN_2X2 flag. If this is encounter during mapgen, the surrounding 4x4 submaps (2x2 overmap terrain) are generated at once.
corrected check for ground level in mapgen.

The above terrain is now air, not empty/invalid/the null-oter.