Add support for AVIF images

[Lonniebiz]

The latest Chrome and Firefox 87 support AVIF images, but if you download and collect these AVIF image files (.avif) Viewnior won't open/display them.

Please add support for AVIF images:

• https://en.wikipedia.org/wiki/AV1#AV1_Image_File_Format_(AVIF)

Compare AVIF images to jpg, and webp:
https://jakearchibald.com/2020/avif-has-landed/

[Lonniebiz]

Downstream: https://bugs.launchpad.net/ubuntu/+source/viewnior/+bug/1917512

[ncmprhnsbl]

viewnior can view .avif if libpixbufloader-avif.so is present.
eg. in archlinux, this is included in the libavif package.
imo, this sort of thing is a distro packaging level decision on hard or optional dependencies, but the documentation should probably at least cover this stuff .. gets a bit tricky with the mimeinfo part of the .desktop file..

[ncmprhnsbl]

further info: libheif (which gimp uses to deal with heif and avif) also provides a pixbuf loader: libpixbufloader-heif.so
it's possible that viewnior(and any other gdkpixbuf based viewer) can use/uses that to view avif ..

[Lonniebiz]

AVIF Browser Support:
https://caniuse.com/avif

Since AVIF images will soon be supported by all web browsers, I think Viewnior should consider "packages that facilitate Viewnior's ability to view avif images" as default dependencies.

Someone who installs Viewnior expects it to be for images, what VLC is for video. When you install VLC you expect it to play any video you throw at it. For me, Viewnior is that type of application but for images instead of video. I especially expect it to display any images supported by web browsers (without further configuration and without separate package installation tasks).

I hope whoever fixes this will also fix webp too.

[hellosiyan]

As explained on https://siyanpanayotov.com/project/viewnior/faq and in #20, Viewnior relies on GDK-Pixbuf and any extra loader modules to handle image format decoding and file loading.
@Lonniebiz I understand your point, and VLC is indeed excellent, but I do not agree there are the same expectations towards Viewnior, which was built to be "small and simple". And I hope you will agree with me that adding extra dependencies to cover "any image you throw at it" does not fit the "small and simple" mindset.