Animated PNG (APNG) read support

[randy408]

APNG is an unofficial extension by Mozilla, it allows for animated PNG files that work similarly to animated GIF files, while supporting 24-bit images and 8-bit transparency not available for GIFs.

• https://wiki.mozilla.org/APNG_Specification
• https://sourceforge.net/projects/apng/
• https://commons.wikimedia.org/wiki/Category:Animated_PNG

This issue depends on #40 (row- and chunk callbacks)
This would be implemented without callbacks, it should be possible to reuse spng_decode_image() for decoding the APNG frames.

Implementation

• Parse and validate acTL, fcTL chunks

• Implement frame decoding / fdAT parsing

• API

Testing

• Create a libpng-apng repository: https://github.com/randy408/libpng-apng (update to v1.6.39) Went with another approach

• Modify libpng's meson.build to enable APNG support

• Replace the current libpng Meson wrap

• Add APNG test images

• Frame and metadata comparison tests against libpng

Other

• Update seed corpus

• Update fuzz target

Documentation

• API Documentation

• Document the new testsuite requirements for distro packagers

• Update migration guide

• Usage example

Depends on #10, #38

[Kochise]

Official Animated PNG format is MNG : http://www.libpng.org/pub/mng/

Reference pictures are here : http://www.libpng.org/pub/mng/mngpics.html

[MCOfficer]

Official Animated PNG format is MNG : http://www.libpng.org/pub/mng

Reference pictures are here : http://www.libpng.org/pub/mng/mngpics.html

Official or not, it is for all intents and purposes dead. It only made it into one major browser, from which it was removed later. APNG is playing in a different league.

In a way, it's the XHTML of png. (no html5-to-apng comparison intended)

[svgeesus]

APNG is now part of PNG Third Edition rather than being documented in a separate extension document.

[mkirkland4874]

Does this branch work for apng reading?? Or still more to be done to decode?