make decomposing leaves no longer traverse nodes that don't affect their
decomposition rate, when determining said rate. this means that they're
only affected by nodes that either touch them directly, or touch nodes
that affect them.
leaves now have a higher chance of turning into dirt, the higher they're
piled within the node. full stacks (taking up an entire node) always
turn into dirt.
decomposing leaves now stack in layers, each one 1/4 of a node in
height.
currently this doesn't affect their behaviour, beyond making them stack
when placed or landing.
make `flood_fill` able to be interrupted if its callback returns false;
the meaning that was previously indicated by this return value is now
indicated by nil instead.
update code that uses this function to account for this change.
