Backport heif-gdk-pixbuf to buster

[Tarik Graba <tarik.graba@telecom-paris.fr>]

Hi everybody,

Is their a chance heif-gdk-pixbuf is added to the Debian 10 backports?

This format is often used by some smartphone owners and actually we need
to convert the image files before visualizing them in gnome.

Thanks,

Tarik