Also update libav->ffmpeg as theres pretty much no code left from libav.
The new code is faster, requires fewer mallocs and less memory. Its
also half the number of lines of code.

This code is not 100% identical in behavior to the previous, but the
differences appear to be rather limitations of the previous design
than intended though i could be wrong of course.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>