Embed subtitles at the same time when merging video and audio

[nyuszika7h]

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Description of your issue, suggested solution and other information

When video and audio has to be merged and --embed-subs is used, the subtitles should be also muxed at the same time as the video and audio, that would be much faster.

[yan12125]

Partial duplicate of #8476. The major difficulty is how to create a command that does multiple jobs at a time and compatible with recent avconv/ffmpeg versions. If you have a good idea, feel free to post there.