Do not decode non reference pictures while prerolling.
It improves seek performance when decoding with ffmpeg.

It should fix/workaround https://bugs.launchpad.net/ubuntu/+source/vlc/+bug/324911
but I am not sure if it is the right way.

Qt: match the access changes for cdda-track specification (commit 55e3dca3 by fenrir )

- Trash Audio
- Keep original Audio

It is cleaner to not send event from caller threadand will fix a deadlock in the
playlist code.

This is ugly but we can do jack about this (short of forking ffmpeg).

avcodec is used from at least three different plugins: avcodec,
switcher (which seems dead though) and indirectly in avformat. We need
to have a common interlock for all of these.

This fixes a crash from the avformat (de)mux.