main: Do not release the lock if it hasn't been acquired

On various systems, trying to release a mutex that hasn't been acquired
will result in a run-time error.

In order to avoid this, we trylock() the Big Clutter Lock™ and
immediately unlock() it, regardless of the result; if the lock was
already acquired, trylock() will immediately fail, and we can release
it; if the lock was not acquired, trylock() will succeed, and we can
release the lock immediately.

This is necessary to maintain binary compatibility and invariants for
Clutter applications doing:

  clutter_init()
  clutter_threads_enter()
  ...
  clutter_main()
  ...
  clutter_threads_leave()

instead of the correct:

  clutter_init()
  clutter_threads_enter()
  ...
  clutter_threads_leave()
  clutter_main()
  clutter_threads_enter()
  ...
  clutter_threads_leave()

With Clutter ≥ 1.12, the idiomatic form is:

  clutter_init()
  ...
  clutter_main()

given that the public Big Clutter Lock™ acquire/release API has been
deprecated, and nobody should take the lock outside of Clutter itself.

https://bugzilla.gnome.org/show_bug.cgi?id=679439

clutter/clutter-main.c

@@ -226,6 +226,16 @@ clutter_threads_impl_lock (void)
 static void
 clutter_threads_impl_unlock (void)
 {
+  /* we need to trylock here, in case the lock hasn't been acquired; on
+   * various systems trying to release a mutex that hasn't been acquired
+   * will cause a run-time error. trylock() will either fail, in which
+   * case we can release the lock we own; or it will succeeds, in which
+   * case we need to release the lock we just acquired. so we ignore the
+   * returned value.
+   *
+   * see: https://bugs.gnome.org/679439
+   */
+  g_mutex_trylock (&clutter_threads_mutex);
   g_mutex_unlock (&clutter_threads_mutex);
 }