[improvement] single execution of ffmpeg instead of 5

[diogosena]

Currently, ffsubsync try to get the longest of the 5 first subtitles of the video, but is doing this using 5 sequential executions of ffmpeg.
This is by far the slowest step, and doing 5 times in a row is very slow, this can be done in one call of ffmpeg using multiples maps and outputs.