Chosen by DRAW_SPRITE_PIXELS from the top three bits of the drawing mode. Used only for doors and furniture; creatures are drawn by DRAW_THING. The eight are the eight ways to lay a rectangle down: 0 as stored, 1 mirrored, 2 turned a quarter clockwise, 3 transposed (mirrored across the diagonal), 4 upside down, 5 a half turn, 6 transposed the other way, 7 turned a quarter anticlockwise. A door's mode is its wall, which is how one picture serves all four.