[webvtt] Merge daisy-chained duplicate cues (#638)

Fixes: https://github.com/yt-dlp/yt-dlp/issues/631#issuecomment-893338552

Previous deduplication algorithm only removed duplicate cues with
identical text, styles and timestamps.  This change also merges
cues that come in ‘daisy chains’, where sequences of cues with
identical text and styles appear in which the ending timestamp of
one equals the starting timestamp of the next.

This deduplication algorithm has the somewhat unfortunate side effect
that NOTE blocks between cues, if found, will be emitted in a different
order relative to their original cues.  This may be unwanted if perfect
fidelity is desired, but then so is daisy-chain deduplication itself.
NOTE blocks ought to be ignored by WebVTT players in any case.

Authored by: fstirlitz

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