Add support for animated HEIC and AVIF when libheif has this feature

[t00350320]

vips copy gif2.gif gif2avif.avif

gif2 has animation effection ,
gif2avif is a static one.

Environment
(please complete the following information)

• OS: [Linux version 3.10.0-1062.el7.x86_64 (gcc version 4.8.5 20150623]
• Vips: [e.g. 8.14.2]

Additional context
Add any other context
gif2.zip
gif2avif.zip

[jcupitt]

Hi again, you need:

vips copy 3198.avif[n=-1] x.avif

By default, libvips only opens the first page of a many page (or many frame) image. Set n to the number of pages to open, or -1 for all pages.

You'll find features like frame time and looping are not supported. When libheif adds these features, libvips will need to enhanced to use them.

I thought there was an open issue for this, but I can't find it :( Let's tag this as an enhancement.

[lovell]

Upstream libheif issue for this at strukturag/libheif#377