Add a 3rd dimension to the map

[aylusltd]

• Add an additional dimension to the world matrix
• Add stairs that can go either up or down to load a different screen from a different level of the map
• Add dungeon screen generation (no grass, trees, etc.)

[aylusltd]

Logic should be pretty straightforward, and a good test of how hard it is for someone else to add sprites. The whole thing was built to make that easy, so if it's not, this would be a great place to find out.