make sure that the node at the placed position is actually a decomposing
leaves variant before doing anything else. (this may not be the case,
since a node may start falling immediately after being placed.)
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 the wrapper `blockgame.register_node` make reasonable assumptions
that make node registration more convenient.
change other code to take advantage of that change.
also remove the `blockgame.reg_simple_node` utility since it's now
redundant.
leaves that aren't in contact with living logs, or are not in contact
with any supported leaves, or are too far away from living logs, will
now die and fall down.
