In the process, we can clean up some of the command line argument
handling. Sadly, argparse's mutually exclusive groups are too
primitive to get rid of the autocrop/manual crop conflict checking,
but we'll have to live with those two lines I guess.
This script apparently also works with Python 3 now, which is neat.
Would crash instead of gracefully exiting if a tileset dir wasn't provided. Might also change how the memory limit option works - currently has to be provided, even though that's not mentioned.