docs: public synchronized static Task queue(Listener callback) {
      final Task task = new Task(callback);
      // TODO: synchronization?
      if(THREAD != null && THREAD.isAlive()) {
        THREAD.queue.add(task);
        return task;
      }
      THREAD = new ThumbnailThread();
      THREAD.queue.add(task);
      THREAD.start();
      return task;
    }