It currently does the following:
1) get a zeroed audio buffer
2) copy some properties (but not the data) of the input buffer to it
3) pass this buffer to the output filter
This looks useless and is indeed not used by any filters, therefore
delete it.
Make ff_null_filter_samples() (just pass the buffer to the next filter)
the new default.
It's redundant, since the input buffer is passed as a parameter to the
filter_samples() callback, and can lead to stale pointers remaining on
the link.
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.