libavfilter API was designed in order to be clarly distinguished from the
libavcodec API, including avcodec.h in avfilter.h is not going to help to
stick to this principle.
The inclusion of libavutil/audioconvert.h in many files was required
because avcodec.h includes audioconvert.h.
libavfilter/avcodec.h is where the lavc/lavfi interface should be
entirely placed.
The planar/packed switch and the packing_formats list is no longer
required, since the planar/packed information is now stored in the sample
format enum.
This is technically a major API break, possibly it should be not too
painful as we marked the audio filtering API as unstable.
