Lighting now works properly in the dungeon, keeping track of everything
visible.  It still does not work for the overworld.  The way it works in
the dungeon is by getting a copy of the current tile, changing the visibility
of that to true, and then passing that back to the set_tile method in the current
chunk.  Another way to do this would be to have a chunk get_tile method return a
pointer, but the problem is that that method relies on the get_tile method in the
overworld and dungeons, which also do not return pointers.  So everything that needs
a pointer would have to be modified.
Merge branch 'master' of https://github.com/schiem/roguelike