Add support for APNG

[oss6]

Summary

• This fixes APNG support #212
• Resources
  • https://wiki.mozilla.org/APNG_Specification
  • https://en.wikipedia.org/wiki/APNG (structure of a single PNG chunk)

Description

The signature of an APNG file is the same as the PNG one.

To be recognized as an APNG, an acTL chunk must appear in the stream before any IDAT chunks - https://wiki.mozilla.org/APNG_Specification

A solution is to check from the end of the IHDR chunk to the first IDAT chunk: if we find an acTL definition in between (61 63 54 4C) then we return APNG.

Note - A PNG/APNG chunk has the following structure: Length (4 bytes) | Chunk type (4 bytes) | Chunk data (length bytes) | CRC (4 bytes) (https://en.wikipedia.org/wiki/APNG).

Notes

• MIME Type: image/apng (https://wiki.mozilla.org/APNG_Specification#MIME_type)
• Extension: apng or png

Thank you very much!