Support for routing generic ancillary data between video side data and subtitle data streams.

It seems like a lot of the discussion around this has revolved around decoding subtitle information and converting between subtitle formats. This is an exciting set of features, but there are use cases that I have not seen explained yet.

Can someone please explain how this patchset would handle situations where an existing subtitle feed of type 608 and 708 closed captions needs to be routed from video side data to VBI or VANC data for a 436M stream or decklink output? This is a use case where it is important that all positioning and formatting information be preserved and decoding to raw timed text or other generic subtitle format is not desirable. I realize this may require an additional patchset to implement in the future, but lets make sure the proper filtering infrastructure is in place for this use case.

CC Input and Output formats
CC 608 and 708 tracks as SCTE-128 in mp4 file as either source or destination
CC 608 and 708 tracks as .scc or .mcc files
CC 608 and 708 tracks contained in mxf OP1A 436m VANC data streams

This may also apply to other data types or control information such as camera control information, engineering automation control data SCTE-35 or SCTE-104 markers or specific data ancillary data formats that FFmpeg has no reason to understand or decode directly but should have the ability to pass and route between container types.