* test/icecream: Add and install a PNG file, and a JPEG file with a wrong .png suffix ("notapng.png"). * debian/control: Add optipng dependency, and optipng/imagemagick build dependencies. (We'll need imagemagick for the test suite.) * test/run: Add test_optipng() for verifying that shipped PNGs are smaller than original and visually identical, and that non-PNGs are shipped unmodified. * pkgstripfiles: Call optipng on all PNG files, update md5sum if successful.