desktop_login: don't use solidify_wallpaper, use a hack instead
    
    solidify_wallpaper only does the current session, and does it in
    a kind of painful way on each desktop. For apps_startstop this
    is kinda okay, but for desktop_login it's slow and error-prone
    to do this three times, every time. Let's replace it with a hack
    that just replaces the actual wallpaper files with a solid black
    PNG file. This only takes effect after a logout, but it should
    affect all logins on all desktops once it's done. So long as
    the base backgrounds package doesn't change layout too much.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>