Don't recurse the mainloop in _gtk_tree_view_column_start_drag().
It doesn't serve any discernible purpose, and recursing the
mainloop from the flush-events phas of the frame clock breaks
frame synchronization with mutter.
https://bugzilla.gnome.org/show_bug.cgi?id=705176
gdk_window_show (tree_view->priv->drag_window);
gtk_widget_grab_focus (GTK_WIDGET (tree_view));
- while (gtk_events_pending ())
- gtk_main_iteration ();
tree_view->priv->in_column_drag = TRUE;