Fix active_object_count and active_object_count_wider not getting updated between ABM calls causing huge amounts of DMs and rats and stuff

Rename menu background to menubg.png, move unknown_block.png and unknown_object.png to client, make generating a placeholder image for the filename "" not cause an error

Make map generator as much threaded as possible (not much benefit with current generator because of small generator chunk size (a single MapBlock))