From dummy.c:

/*
 *  Very simple program, that has been used by us
 *  (V. Babin <babin@users.sourceforge.net> and
 *   S. Kondrat <valiska@users.sourceforge.net>)
 *  during toying with libmng (http://www.limng.com)
 *
 *  License: GPL :-))
 *
 *  7 July 2001:  added key-press/button-press handling to exit viewer without
 *                window-manager help; added libmng version info [Greg Roelofs]
 */

From gtk-mng-view.c:

/* Toy widget for GTK+ for displaying MNG animations.
 *
 * Copyright (C) 2000 The Free Software Foundation
 *
 * Author(s): Volodymyr Babin <vb@dwuj.ichf.edu.pl>
 *
 * This code is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */