cccombiner: implement scheduling

Prior to that, cccombiner's behaviour was essentially that of a funnel: it strictly looked at input timestamps to associate together video and caption buffers.

This patch instead exposes a "schedule" property, with a default of TRUE, to control whether caption buffers should be smoothly scheduled, in order to have exactly one per output video buffer.

This can involve rewriting input captions, for example when the input is CDP sequence counters are rewritten, time codes are dropped and potentially re-injected if the input video frame had a time code meta.

Caption buffers may also get split up in order to assign captions to the correct field when the input is interlaced.

This can also imply that the input will drift from synchronization, when there isn't enough padding in the input stream to catch up. In that case the element will start dropping old caption buffers once the number of buffers in its internal queue reaches a certain limit (configurable).

The property is exposed so that existing users of cccombiner can revert back to the original behaviour, but should eventually be removed, as that behaviour was simply inadequate.