* First parameter is the video image.
* Second parameter is true if the image is the same as the last one that was emitted.
* Third parameter is either 0 or a subtitle that should be on this frame.
+ * Fourth parameter is the time relative to the start of this source's content.
*/
boost::signals2::signal<void (boost::shared_ptr<const Image>, bool, boost::shared_ptr<Subtitle>, Time)> Video;