overlays can now be assigned to a pure icon state. You might have some
sort of special overlay (like clothing) that is specific to each different
icon. This would allow you to put that overlay inside the object's .dmi
icon and display the overlay by simply doing, for instance, overlays += "armor"
(where "armor" is an icon-state of the current object's icon).
The limitation of this method over using a separate .dmi file is
that the overlay cannot have its own set of icon states, but when you don't
need that level of complexity, this is a nice compact way of packaging
things and is consistent with the ability to use icon states in flick().
This same technique also applies to image().