Polari still runtime depends on GTK3 for thumbnailer

src/thumbnailer.js used by urlPreview.js via separate gjs call still uses GTK3 explicitly:

import Gdk from 'gi://Gdk?version=3.0';
import Gtk from 'gi://Gtk?version=3.0';

Looks like a bunch of distros haven't noticed that and have removed the gtk3 deps, thus not guaranteeing thumbnailing by urlPreview will actually work (though gtk3 GIR is probably installed for something else for most users and thus be fine in practice for those users).