qtdemux: Split CEA608 buffers correctly so that each output buffer represents a single frame

Merged Sebastian Dröge requested to merge slomo/gst-plugins-good:qtdemux-cc-fixes into master

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.

CC @bilboed

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