Extract thumbnails using ffmpeg.
Behavior is controlled by three new preferences fields:
- extract_video_thumbnails (bool): if true, thumbnails are calculated.
- extract_video_thumbnail_position (int 0..100): position in video
where thumbnail is fetched.
- ffmpeg_executable (string): path of ffmpeg executable.
If ffmpeg refuses to start, extract_video_thumbnails is set to false
to avoid unnecessary churn.
Video thumbnails are marked by an overlay.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
