libpng-1.6.18 and 1.5.23 released

libpng-1.6.18 and 1.5.23 were released last week. Due to the outage, they aren't available yet in the SourceForge File Release System. They are, however, available from the glennrp/libpng-releases repository at github. This is a cleanup release that fixes some harmless Coverity defects and removes some unused code.