Sync icon validation code with flatpak
Sync icon validation code with flatpak

Set up the sandbox in the same way as flatpak build-export
will do in the future, and pass the max. size as arguments
to the icon validator.

This will make it easier to switch to using a single
validator binary when flatpak gains one.

matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal