When a new mapblock is generated and the mod checks the neighbors around
a target to place a fallen twig, if it finds an unknown node (because
it's in a neighboring, old mapblock from a previous session -- perhaps
an old moss node that hadn't converted-over to wallmounted yet), trying
to check its buildable_to state will fail, since that requires that
there be a node def to look at, which an unknown node wouldn't have.
This substitutes a known not-buildable_to node for those cases, so that
the code won't try to overwrite what it found.
I couldn't use leaf decay to make moss disappear when a trunk is dug,
because it breaks leaf decay on that tree's leaves: the leafdecay
function is not a true "register"- type function that can be run more
than once on a given trunk node, it's an all-or-nothing override and
only the last call for any given trunk actually sticks.
Since moss is... was facedir, attached_node didn't work right either, as
it doesn't have a mode to look for a vertical surface behind the
attached object (like how it works with wallmounted items), so this
converts moss to true wallmounted and uses attached_node like I
originally wanted.
To avoid losing the effect where moss can be rotated randomly when
generated, I registered 4 nodes for each moss type, with
increasingly-rotated textures.
