fontchooserwidget: use appropriate model for GtkTreeIter

We were previously mixing the model used when filtering with an iter that
has been resolved to the backing model.

This results in both an invalid row index as well as an invalid
iter->stamp.

gtk/gtkfontchooserwidget.c

@@ -462,7 +462,7 @@ cursor_changed_cb (GtkTreeView *treeview,
   gtk_tree_model_filter_convert_iter_to_child_iter (GTK_TREE_MODEL_FILTER (priv->filter_model),
                                                     &iter,
                                                     &filter_iter);
-  gtk_tree_model_get (priv->filter_model, &iter,
+  gtk_tree_model_get (priv->model, &iter,
                       FONT_DESC_COLUMN, &desc,
                       -1);