Add support for AVIF images

[Lonnie Lee Best]

Please add support for AVIF image file format:
- https://en.wikipedia.org/wiki/AV1#AV1_Image_File_Format_(AVIF)

The latest Chrome and Firefox 87 support AVIF images, but Viewnior won't open/display them.

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

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: viewnior 1.6-1build2
ProcVersionSignature: Ubuntu 5.4.0-66.74-generic 5.4.86
Uname: Linux 5.4.0-66-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Tue Mar 2 12:59:24 2021
InstallationDate: Installed on 2020-04-26 (310 days ago)
InstallationMedia: Kubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: viewnior
UpgradeStatus: No upgrade log present (probably fresh install)

[Lonnie Lee Best (launchpad-startport)]

Upstream? : https://github.com/hellosiyan/Viewnior/issues/95

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Paddy Landau (paddy-landau)]

This bug report is about a specific app, Viewnior, but AVIF support should be generic to all of Ubuntu. For that reason, I'll mark this as a duplicate of #1946567.