Caves of WIP

Added a underground layer that spans the whole map: when exploring you can find entrances or ladders that let you access a cave system (ignore that is raining undergound). 

They are generated with a different algorithm based on biomes, i used for now cellular automata, drunken walk and Voronoi diagrams to carve a space into the map, then i extrude south all the borders to create walls, still need to figure out how to place autotiles and let the engine set their direction after map load.

Also some adiacent biomes are connected with a open border so you can have caves spanning several maps.

By making some research i don't think there is a way to programmatically insert engine shadow like it happens automatically when drawing the map, so i'll add a shadowed half tile on the right of walls and ceiling.