bug#37153: 26.1; some png images scrambled

[Roland Winkler]

On Sat Aug 24 2019 mituharu@math.s.chiba-u.ac.jp wrote:
> I suspect there is a longstanding typo (or thinko) in PNG
> transparency handling code.

It may be helpful to check the "official" test-suite for PNG

http://www.schaik.com/pngsuite2011/pngsuite.html

which contains also png files with transparency

http://www.schaik.com/pngsuite2011/pngsuite_trn_png.html

Somehow emacs ignores the transparency for image type png, see for
example the file tm3n3p02.png with multiple levels of transparency.

Transparency is honored for these images when emacs uses imagemagick.