NOT use batch_dump_frames.py with -n > 1

[escorciav]

It seems that FFmpeg is not thread-safe even spawned in different contexts.

• This might affect other scripts that call FFmpeg such as batch_video_editing.py and video_info.py. However, I have only seen the bug in batch_dump_frames.py.

[escorciav]

@ soldelli pointed out this explanation as cause. However, it's unclear to me if subprocess is doing something along those lines.

@ dandelin noticed another issue using FFmpeg+joblib. The solution was to use only multiprocessing. However, joblib uses multiprocessing as the default backend 😖