qtdemux: Extract CEA608 framerate from the (first) video stream
EA608 closed caption tracks are a bit special in that each sample can contain CCs for multiple frames, and CCs can be omitted and have to be inferred from the duration of the sample then. As such we take the framerate from the (first) video track here for CEA608 as there must be one CC byte pair for every video frame according to the spec. For CEA708 all is fine and there is one sample per frame.