Bug#706798: transition: Libav 9

[Moritz Muehlenhoff <jmm@inutil.org>]

> > Since libx264 is largely subsumed by libav, I think it makes very
> > much sense to do them both at the same time. I am convinced that we
> > will have less effort this way.
> > 
> OK, feel free to go ahead with both.

(As discussed with Reinhard)

Please add

remove mplayer/2:1.0~rc4.dfsg1+svn34540-1 

Mplayer needs more changes to be compatible with current libav and has problems with
other external libs (#708140). It can be fixed later or maybe replaced by
mpv/mplayer2.

Cheers,
        Moritz