Feature request: PNG support

[finnbear]

I'd like to have the option to write IPTC metadata to PNG files.

(some time in the future, I'll investigate how hard this is to implement and, if I'm able, submit a PR) Nevermind, I've realized I need to use XMP for WebP support.

See also #2.

[finnbear]

There are no standard for Exif, IPTC data. In Exiv2, when Exif, IPTC are added, they are stored in zTXt text chunks and save as ASCII.

(https://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_PNG_files)

This is out of date since PNG now supports EXIF. AFAIK, it still applies to IPTC. I couldn't find explicit information on what PNG IPTC format Google Images expects.

Here's an example of a chunk GIMP writes when saving a PNG with IPTC:

(https://www.nayuki.io/page/png-file-chunk-inspector with this test image: )

That IPTC data recognized by https://getiptcpmd.nitsvc.net/getiptcpmd.html

[alvinometric]

A lot of this library is inpired by https://github.com/TechnikTobi/little_exif which does support PNG, might be worth having a look at how he did it?