Fix 'libpng error: IDAT: invalid distance too far back' error

The update to libpng 1.6.3 cause some PNGs to be unreadable or wrongly displayed.
See https://bugzilla.mozilla.org/show_bug.cgi?id=841734#c9 and https://mailman.archlinux.org/pipermail/arch-dev-public/2013-May/024872.html for more.

In virtuoso this only happens to opts_16.png which is wrongly displayed: http://wstaw.org/m/2013/05/05/plasma-desktopU18187.png