Missing const in PixelGraph

There is a clear violation of const-correctness in pixelgraph.h:

    inline PixelGraph::PixelGraph(Glib::RefPtr<Gdk::Pixbuf const> pixbuf)
    // ...
    guint8 *pixels = pixbuf->get_pixels();

Since the pixbuf is const, the last line above should be

    guint8 const *pixels = pixbuf->get_pixels();

This issue breaks building libdepixelize with GTK4.