Partially Fixed Exit Doors

Added code so that exit doors will search for a destination and place a
platform for the player on the other side. However, the search function
only searches upward, not downward. I'd like to discuss some details
about this before completing the implementation:
1. How should the search work?
2. What should be the likelihood of a dungeon exit leading to a
different root dimension than the dungeon's actual root?

I've tested the exits and they're working well.