Assume <image>foo</iimagemg> is a source image kind for AppData files
Assume <image>foo</iimagemg> is a source image kind for AppData files

It doesn't make sense to have type="source" for AppData files as they are all of
type SOURCE, so just assume this is the default.

This fixes screenshots for any package using v0.6+ AppData files.

mitya57/appstream-glib,ikeydoherty/appstream-glib,ximion/appstream-glib,superm1/appstream-glib,superm1/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,kalev/appstream-glib,ikeydoherty/appstream-glib,ximion/appstream-glib,kalev/appstream-glib,mitya57/appstream-glib