[tex4ht] disable dvipng on preexistent png images?

[Michal Hoftich]

Hi Giuseppe,

> After some upgrades, I've a problem during htlatex compiling.
> It seems that dvipng also wants convert already existing png images...
> Also, during the conversion I get this warning: dvipng warning: at (20,732) unimplemented \special{em: graph icons/pcore/project_criterion.png,72bp,72bp} ]
> on all png images.>

this issue was fixed in TeX4ht sources some time ago, but it seems
that Miktex still doesn't have correct files updated.
I think it can be fixed using this answer:
https://tex.stackexchange.com/q/517859/2891.

Best regards,
Michal