Support pngs with more than 8 bits

[NicolasHug]

We support 1-bit pngs and 8-bits pngs, but we explicitly don't support 16-bits or 32-bits pngs (see #3613). It would be good to add support for those images.

[saswatpp]

Hi @NicolasHug Can I take this issue/enhancement? I hope this is not super urgent because I have to take time to understand internals before coming up with a design and a PR.

[NicolasHug]

Thanks for your interest in this issue @saswatpp , it isn't super urgent, don't worry.

I'm not sure how difficult this one could be though, it's the kind of thing that can either be a matter of adding a few lines of code... or a few hundreds. However note that it will definitely involve some C++ coding, so it's good to have prior experience with C++.

Perhaps @andfoy has a better idea of the expected difficulty here?

[NicolasHug]

@saswatpp just FYI I started working on this in #4657