Support for AVIF image format

This is discussed in the forum, starting here:

https://forum.puppylinux.com/viewtopic.php?p=160784#p160784

The 'shared-mime-info' package does not recognize the mimetype "image/avif" so I have added it in /usr/share/mime/packages/avif.xml

Image viewers such as Viewnior rely on gdkpixbuf plugins, and I have added 'libavif-gdk-pixbuf' package builtin.

mtPaint does not recognize avif images, Gimp does. Right-click on a .avif image and Gimp will be offered to open it. Left-click and it will open in Viewnior.   

Tags: easy