See URL for detailed changes since libpng-1.5 I suggest adding >=libpng-1.6.0 p.masked for testing.
By the way... wouldn't this be the perfect opportunity to introduce subslots into libpng ebuilds?
yes, this shouldn't be added to tree without subslotting if noone beats me to it, i'll add this subslotted as 0/16 (assuming the SONAME is 16) with empty keywords for testing but there are many libpng reverse deps in portage using := which is wrong when they should be :0= instead to avoid conflicting with the binary-only slot 1.2
# qlist -C libpng | grep \.so\. /usr/lib64/libpng16.so.16.1.0 /usr/lib64/libpng16.so.16 SONAME is 16 as you assumed. Right now I have revdep-rebuild running on two of my machines and up to now I only found sci-astronomy/celestia to break with png16. This looks promising.
(In reply to comment #3) > # qlist -C libpng | grep \.so\. > /usr/lib64/libpng16.so.16.1.0 > /usr/lib64/libpng16.so.16 > > SONAME is 16 as you assumed. > +*libpng-1.6.1 (05 Apr 2013) + + 05 Apr 2013; Samuli Suominen <ssuominen@gentoo.org> +libpng-1.6.1.ebuild: + Version bump wrt #464728 by Lars Wendler. Include subslotting. Temporarily + without KEYWORDS for testing. > Right now I have revdep-rebuild running on two of my machines and up to now > I only found sci-astronomy/celestia to break with png16. This looks > promising. pngquant failed here