bug#30670: 26.0.91; PNG files on Windows

[Eli Zaretskii]

> From: Alex Schröder <kensanata@gmail.com>
> Date: Fri, 2 Mar 2018 09:45:53 +0000
> Cc: 30670@debbugs.gnu.org
> 
>  How do you invoke Emacs from that installation?
> 
> From a shortcut in my startmenu.
> 
> Target: "C:\Portable Programs\emacs-26.0.91-x86_64\bin\runemacs.exe"
> Start in: %HOMEPATH%
> 
> %HOMEPATH% is set to \Users\asc
>  
>  Are there any other versions of libpng16-16.dll on that system?
> 
> I ran the following:
> $ find.exe / -name libpng16-16.dll
> /cygdrive/c/Portable Programs/emacs-26.0.91-x86_64/bin/libpng16-16.dll
> /cygdrive/c/Portable Programs/inkscape/libpng16-16.dll
> /cygdrive/c/Program Files/emacs-25.1-x86_64-w64-mingw32/bin/libpng16-16.dll
> 
> I found the old C:\Program Files\emacs-25.1-x86_64-w64-mingw32\bin on my 
> path, removed it and restarted
> Emacs, verified that it was not listed in (getenv "PATH"), but no luck. I 
> still cannot display PNG images.
>  
>  Does pngfix.exe work (it should be in the same directory as
>  libpng16-16.dll)?
> 
> Yes it does.
> 
> $ ./pngfix ../lib/python2.7/idlelib/Icons/idle_16.png
> IDAT OK  maximum 12 12 1070 2064 ../lib/python2.7/idlelib/Icons/idle_16.png 
> 
>  And finally, what is the value of libpng-version in your Emacs?
> 
> libpng-version
> 10634

Strange.  I can only suggest stepping with a debugger through
init_png_functions, to see what exactly fails there and why.