Support APNG

[retep998]

This issue is to track the state of Animated Portable Network Graphics support.

• Animated Portable Network Graphics support.
• Introduce an API for animated images, see Proposed API for animated images #145

More information:
https://wiki.mozilla.org/APNG_Specification

[nwin]

@retep998 are you planning working on this?

[retep998]

@nwin Unfortunately no. I only filed an issue because APNG support is something I'd like for my own projects and this seems like the perfect library to handle that.

[abonander]

cc image-rs/image-png#93

I've run into a use-case for this. I'm generating some demo animations for img_hash but they're not really suited for GIF encoding for a number of reasons. I'd mostly just like an easy way to losslessly export a sequence of frames to a format that FFMPEG can ingest.

[HeroicKatora]

After long waiting we finally have #1250 🎉 It needs a small amount of precursory work in png but has already been tested to work with a modified build of emulsion.