Extract all subtitle streams simultaneously

Extracting a subtitle stream is a disk I/O bottlenecked operation as
ffmpeg has to read through the whole file, but usually there is nothing
CPU intensive to do.

If a file has multiple subtitle streams, and we want to extract more
of them, extracting them one-by-one results in reading the whole file
again and again.

However ffmpeg can extract multiple streams at once.

We can optimize this by extracting the subtitle streams all at once
when only one of them gets queried, then we will have all of them
cached for later use.

It is useful for people switching subtitles during playback.

It is even more useful for people who extract all the subtitle streams
in advance, for example with the "Subtitle Extract" plugin.
In this case we reduce the extraction time significantly based on the
number of subtitle streams in the files, which can be 5-10 in many
cases.

Signed-off-by: Attila Szakacs <szakacs.attila96@gmail.com>

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