Add support for APNG

[lpatiny]

Add the support for animated PNG.

decodeToBeDefined that returns systematically an array of images.

[opatiny]

Do we want to decode or generate APNG (or both)?

[targos]

Decode.

[EscapedGibbon]

It seems like all is needed is to add an additional chunk to mark that an image is in fact APNG, then it just stacks chunks with data in a sequence. So it can be decoded as an array of frames. Probably current data decoding function will work on APNG data frames as well.

[targos]

Sounds simple enough! It looks like we can create a separate APNG decoder, that will internally call the PNG decoder with each frame.
I would would be the decodeAPNG function return an object, so we can have top-level metadata and the array of frames.