Support for APNG format

[K0-RR]

Hello, it seems APNG images aren't animated, just a static frame is displayed.

[jcupitt]

Hi @K0-RR, that's right, libvips has no APNG support right now, because libpng still hasn't integrated this.

I think this is the main libpng thread on APNG:

pnggroup/libpng#267

When libpng has official APNG support, libvips should get it fairly soon after, and then (hopefully!) vipsdisp.

[kleisauke]

According to pnggroup/libpng#735, the upcoming libpng 1.8.0 release will include APNG support.

[jcupitt]

Oh, great! I'll make a libvips issue to add support.