This version is quite broken...

This affects Opening of DVB and BDA.

This commit add some options as safe, based on a search
of issues on the forum.

Additions are:
- ffmpeg-pp-q (prosprocess quality)
- ffmpeg skiploopfilter (speed)
Signed-off-by: Laurent Aimar <fenrir@videolan.org>

Add the ability to change those options:
- video-title-show
- video-title-position
- video-title-timeout

Patch by Anthony Loiseau (thannoy _AT_ actech-innovation _DOT_ com)

 Always reject plugins that have a 0 priority but did not match the
shortcut list (being empty or not).
This makes 0 priority plugin only selectable using a shortcut (which I
think it is what we want).

avcodec will now output extradata using the same format than flac encoder.

Previously, it was possible only on the first subtitle.
The drawback is that we may allow selecting channels that are empty.

since we may want sub-second granularity.

That plus the fact that the decoder already limit its fifo will allow
to mark all --*-cache options safe.

It will prevent #2706 from broken demuxer/streams.

It closes #2706.

The sequence header/entry point are not repeated yet.