[WIP] Car roofs

Show car roof when not boarding it.
Added as pool request mostly to get feedback. Requires a little change in z_orders system, because roof mustn't be displayed upon external door tiles.

Also, a little planning mistake arises: all car trunks are treated as outside a vehicle, i.e. when you step into any car's trunk, you see roof instead of car's insides (unlike when standing on car's open door)