FR: don't obscure stairs

[rigrig]

This Swamp entrance has a cloud of mist on top of it. While this looks interesting, it would be nice if the stairs themselves were excluded from having mist placed on top of them, so you can actually find them.
So instead of this:

  Code:

P.#8.>.8..##...§§§..............#
........#...#§.§§§...............
..##...##.....§§§§§..............
#####'###....§.§@...........#.#..
###.....##...§.§.§..........#....


You would always see this:

  Code:

P.#8.>.8..##...§§§..............#
........#...#§.§§§...............
..##...##.....§§§§§..............
#####'###....§.>@...........#.#..
###.....##...§.§.§..........#....