qtdemux: Extract CEA608 framerate from the (first) video stream

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.