Dropper tile also raises the exit door

[TCH68k]

Steps to reproduce:

1. Change the exit's trigger tile from raiser to dropper.
2. Step on it.
3. Exit will raise.

Illustration video: http://oscomp.hu/depot/sdlpop_drop_event_raises_exit.mkv

This is rather a conceptional flaw, than a bug: it should work as it does with gates, it should shut already opened exits. (I know there is no animation for that, as the shutting door has no background stairs, but the opening animation can be played backward.)

Note: The same happens in the original game, but i still think, this should be fixed.