Improves performance.

First improvement: the queue now avoids doing opposite tasks.
Second improvement: the queue has been split in an urgent one and a lazy one (new displayed sectors=urgent).
 Third improvement: "World.is_exposed" is now cached.
Reduces cacti width and enlarges is_exposed.
Merge https://github.com/boskee/Minecraft
Fixes tree distribution.
Merge commit '5b221b72ba7dfb0a847282d7a8b454041b30424f'