public CrossfadeIcon (Gdk.Pixbuf primary, Gdk.Pixbuf secondary)
{
if (primary == null)
throw new ArgumentNullException ("primary");
if (secondary == null)
throw new ArgumentNullException ("secondary");
this.primary = (Gdk.Pixbuf)primary.Clone ();
this.secondary = (Gdk.Pixbuf)secondary.Clone ();
}