Fix overlapping burnt-in subtitles in some cases (#959).

Firstly, when finding subtitles that exist during a period, only
return those which overlap more than half the period.  This means
that, in a fight over a frame, the longest-running subtitle in that
frame will win.

Secondly, make SubtitleDecoder::get pick the wanted subtitles from
the cache simply by comparing their periods to those that were
requested.  I think this is nicer than what was there before
(basically reevaulating 'what subtitle(s) for this period') and
also makes the first part of this commit effective.

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