The previous behavior using `eof_action=endall` and `shortest=1` would
end the video stream if one of the input stream (video, subtitle) ends.
In some cases the duration of the overlayed subtitles is shorter than the
video stream, causing the output to end when the subtitles end and dropping
the remaining video stream.
This commit changes this behavior so `eof_action=pass` is used instead,
which continues passing the video stream through even if the subtitles end
earlier [1]. `shortest=1` is also removed, as this option implies
`eof_action=endall`.
If the subtitle stream has a higher duration than the video stream, the output
will also end with the video stream without `shortest=1`, as the video stream
is the primary input to the `overlay` filter.
Fix#10698
[1] https://ffmpeg.org/ffmpeg-filters.html#Options-for-filters-with-several-inputs-_0028framesync_0029