mpegtspacketizer: memcmp potentially seen_before data

Theoretically the version number is incremented every time there's a new
section, but in a world of streaming we can't easily make that
assumption.

An example of a broken use case is when we're cat-ing two mpeg-ts files
together, which is equivalent of capturing a DVB stream while switching
channels. A set-top box would know that we switched the channels and
reset the demuxer, but in practice this might not happen.

Part-of: <!2555>
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