decomposition fix.

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.

mods/bg_tree/decompose.lua

@@ -64,6 +64,8 @@ blockgame.register_increasing_abm({
 				score = score + gain
 				return true
 			end
+
+			return false
 		end, 4)
 
 		return data.value + score