Fix an issue where PNG images cannot be loaded on Ubuntu

[nsfisis]

Problem

The latest CI failed because SATySFi failed to load PNG images on Ubuntu.

https://github.com/gfngfn/SATySFi/actions/runs/12861909507/job/35855999813

SATySFi manually built on Ubuntu failed to load PNG too.

Cause

It is because camlimages, which provides functionalities to load PNG images, was built without libpng.

As conf-libpng is optional for camlimages, libpng (technically, libpng-dev apt package on Ubuntu) is not installed by opam install . --deps-only in CI.

https://gitlab.com/camlspotter/camlimages/-/blob/83c53c196fc1e7766ed5c636c3f1d2418ce41293/dune-project#L27-38

 (depopts
  lablgtk
  graphics
  conf-libpng
  conf-libjpeg
  ; conf-libexif
  ; conf-libtiff
  ; conf-libxpm
  conf-libwebp
  conf-freetype
  conf-libgif
  conf-ghostscript))

Solution

Add conf-libpng to package.depends of dune-project.

P.S. I am completely new to OCaml and its ecosystem and am not sure about the correctness of the changes.

[nsfisis]

CI still failed:

Accoding to the build log, opam install . --deps-only did not install conf-libpng:

[EDIT] It was because I did not update satysfi.opam. I'll fix it soon.
[EDIT] fixed