Catch errors from libpng

[akerbos]

There has been a regression/change in libpng that caused pdflatex to throw some errors when including PNG images. These were not caught by ltx2any but dumped directly to shell.

Track down the reason for that and fix it!

(Probably related, similar issue: output from texmf if a font is used for the first time is also dumped to shell. Don't we catch any output from subprocesses of pdflatex?)