The perceptualdiff program was written by Hector Yee with contributions
from Scott Corley and Mick Weiss. It is hosted at http://pdiff.sourceforge.net
The source code added here was obtained by:
svn co https://svn.sourceforge.net/svnroot/pdiff/trunk pdiff
which gave revision 22 of the source code.
The perceptualdiff program is available under the terms of the GNU GPL,
so I've added a note to COPYING about this program, (and the fact that
it is auxiliary only, and does not affect the license of the implementation
of cairo itself).
