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.

tests/interactive/test-texture-async.c

@@ -120,8 +120,6 @@ test_texture_async_main (int argc, char *argv[])
 {
   gchar *path;
 
-  clutter_threads_init ();
-
   if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
     return 1;
 
@@ -140,7 +138,9 @@ test_texture_async_main (int argc, char *argv[])
                                     task, path,
                                     cleanup_task);
 
+  clutter_threads_enter ();
   clutter_main ();
+  clutter_threads_leave ();
 
   g_free (path);