update v15.6

The "Metropolis" update series will include tons of new buildings and elements for your city,  with the goal to supply a complete and realistic city tileset  

A little digression to empower terrain tilesets

- Added a new inclosed grass tileset (with the borders inside and outisde the tilesize)
- Added a grass tileset without anything under it and some new diagonal tiles to the old terrain tilesets

See you and happy game-making! :)