Allow reading of 'avis' ftyp/filetype/brand images

[lovell]

Hello, libheif already has some support for HEIF images with an ftyp of "avis" e.g. in the heif_main_brand function:

libheif/libheif/heif.cc

Lines 184 to 186 in 63be81f

	else if (strcmp(brand, "avis") == 0) {
	return heif_avis;
	}

This PR extends that support to allow these images to be decoded (or at least progress to the next step of validation).

An example AVIF image that currently fails but which this PR will allow can be found at
https://colinbendell.github.io/webperf/animated-gif-decode/2.avif

[hjmallon]

When I tried this the other day I found that this avis files have no primary image, which didn't fit in with how libheif worked. If that is true and the cause of the 'next step of validation' failure then it seems fair to blacklist the brand as they are not supported?

[farindk]

Yes. Let's postpone this until we have proper sequence support. I'm still not sure how important sequence support is, though.

[hjmallon]

I assume people have a use for it, and it is part of HEIF so it seems like it should be part of libheif. Maybe there shoudl be an open issue for 'Support HEIF Image Sequnces (msf1 / avis / etc)' or something to track interest?

[lovell]

I would be so very happy for the whole concept of "animated images" to disappear entirely and become the videos they deserve.

Some avis images in the wild, such as the example above, do appear to store the first frame in the primary item and can be parsed by libheif.

However I will apologise for kowtowing to these and promise never to speak of this again.