Reinstate sponges spawning as mature colonies, and match mapgen
colonies to expected end-state of natural regrowth. This should
make it harder to distinguish an "old growth" area where players
have been hanging around a long time from pristine sponges.
- Inhibit sponge colony growth beyond a maximum size. Players
will have to prune colonies (or, smarter yet, split them) to
grow more sponges.
- Allow sponge colony transplantation. Sponges near the "core"
of a colony (i.e. surrounded by more sponges) are more viable
and more able to survive the player's crude harvesting.
