Partial backport of:

commit 345d865ac6b2f7760503ab144dd68e6062438b80
Author: Matthias Clasen <mclasen@redhat.com>
Date:   Wed Jan 18 00:57:20 2012 -0500

    GtkScale: Fix late setting of marks

diff --git a/gtk/gtkscale.c b/gtk/gtkscale.c
index 4317523fb8f1918a..a573b2f6ba203b92 100644
--- a/gtk/gtkscale.c
+++ b/gtk/gtkscale.c
@@ -1471,7 +1471,7 @@ gtk_scale_add_mark (GtkScale        *scale,
   mark->position = position;
  
   priv->marks = g_slist_insert_sorted_with_data (priv->marks, mark,
-                                                 (GCompareFunc) compare_marks,
+                                                 compare_marks,
                                                  GINT_TO_POINTER (
                                                    gtk_range_get_inverted (GTK_RANGE (scale)) 
                                                    ));