create a constant that represents the amount of tiles a box extends out
from its center in all 4 directions. compute the box size and box center
position constants from this new constant, instead of having them be
hard-coded.
previously, world generation would represent empty tiles as the number
0, while other code would expect them to be an object with its `type`
property set to `"empty"`. the object representation is now used
consistently.
previously, serialization of world data couldn't handle boxes, since
they contain references to their parents, meaning worlds are cyclic.
this commit fixes this, and changes world loading to correctly re-assign
parents to boxes after deserializing them.
this bug is what happens when you commit without testing for more than a
few seconds.
HyperOnion