函数 av_thread_message_queue_set_free_func 

pub unsafe extern "C" fn av_thread_message_queue_set_free_func(
    mq: *mut AVThreadMessageQueue,
    free_func: Option<unsafe extern "C" fn(msg: *mut c_void)>,
)

Set the optional free message callback function which will be called if an operation is removing messages from the queue.