prefs: Don't try to use actual monitor ratio
    
    The current way for (maybe) figuring out the ratio relies on
    deprecated API that is no longer available in GTK4.
    
    The new API works properly on wayland, but is tied to surfaces
    which makes it fiddlier to use. Considering that matching the
    output ratio doesn't really provide any value, just show the
    preview at a fixed ratio of 16:9.