Bug#1017450: eom: Viewing webp images

[Jeremy Bicha <jeremy.bicha@canonical.com>]

There are 3 things needed for eom to fully support these formats.

- They ought to have a gdk-pixbuf loader
- They ought to have a thumbnailer
- The mimetypes needs to be listed in the .desktop

I'll add webp-pixbuf-loader to the Depends. That binary package has
both the gdk-pixbuf-loader and the thumbnailer (sometimes, there would
be a separate Debian package for one).

But neither avif or heif are listed in the MimeTypes currently. You
may want to report that upstream or submit a merge proposal there.

Thank you,
Jeremy Bicha