Apng decoder

[Kurnihil]

Hi, i've extended your library with apng decoding
https://github.com/Kurnihil/png-decoder

Since it was written for fontdue, that likely doesn't need apng capability i have put everything behind a non default features as not to increase fontdue code/import size.

If you are interested i can:

1. comment the file
2. open a pull request
3. collaborate

Anyway, thanks for your previous works

[bschwind]

Hey, that's awesome!

Let's go ahead and open a PR for this as I'm sure it will be useful for others. I'd agree a feature flag is the way to go here. Once you open a PR I'll read through the animated PNG spec and try to leave some comments, as I intentionally skipped that part of the spec the first time I went through. Thanks for your work!