Used by the routines at DOOR_NEEDS_KEY, DOOR_SERF and ACG_DOOR.

Clear bit 3 of the drawing mode -- the bit that says a door is shut, which TEST_ROOM_BOXES reads to decide whether the doorway lets the player through -- and clear it on the far side too, so the two halves never disagree about whether the door is open. DOOR_NEEDS_KEY calls it when the player is carrying the right key, the character doors when the right character walks up, TIMED_DOOR_SHUT's toggle, and ACG_DOOR.