Deprecate clutter_threads_init()

GLib deprecated g_thread_init(), and threading support is initialized
by GObject, so Clutter already runs with threading support enabled. We
can drop the clutter_threads_init() call requirement, and initialize the
Big Clutter Lock™ on clutter_init(). This reduces the things that have
to be done when dealing with threads with Clutter, and the things that
can possibly go wrong.

clutter/clutter-main.h

@@ -107,7 +107,10 @@ gulong           clutter_get_timestamp              (void);
 gboolean         clutter_get_accessibility_enabled  (void);
 
 /* Threading functions */
+#ifndef CLUTTER_DISABLE_DEPRECATED
 void             clutter_threads_init                  (void);
+#endif
+
 void             clutter_threads_enter                 (void);
 void             clutter_threads_leave                 (void);
 void             clutter_threads_set_lock_functions    (GCallback enter_fn,