Fixed a bug in going up and down between dungeons.  The bug was caused
by the buffer being written in the dungeon and not being updated going
up/down stairs.  I changed it so that the buffer only contains the
overworld, as (as we have it now) we don't need a buffer for the dungeon.
However, if we ever make larger dungeons which go over multiple chunks,
it would be necessary to make the buffer for both.  This will also require
treating going between the overworld and dungeon as changing chunks (call
update_buffer).