gtk_init(): initialise the GDK global GPeriodic

Initialise the GDK global periodic clock object from gtk_init() if
nobody installed one before us.

gdk/gdk.c

@@ -823,6 +823,9 @@ gdk_enable_multidevice (void)
  * you should check with gdk_threads_get_periodic() to ensure that it is
  * %NULL before attempting to call this function.
  *
+ * gtk_init() will set the default clock if it has not already been set,
+ * so you need to call this function before you call gtk_init().
+ *
  * Since: 3.0
  **/
 void

gtk/gtkmain.c

@@ -1068,6 +1068,9 @@ gtk_init_check (int	 *argc,
   if (!gtk_parse_args (argc, argv))
     return FALSE;
 
+  if (gdk_threads_get_periodic () == NULL)
+    gdk_threads_set_periodic (g_periodic_new (60, GDK_PRIORITY_REDRAW));
+
   return gdk_display_open_default_libgtk_only () != NULL;
 }