This was required by vf_pp, which was just removed.
vf_dlopen has this stuff in its API. This API is considered stable, so
the related fields are not removed from it. But the fields are always 0
now, so there's no point in keeping the example program around.
vf_pullup.c did some extremely awkward passthrough of this information,
but didn't actually use it.
This is done by requesting a buffer from the next filter in the chain, instead
of always allocating our own. This allows the next filter to e.g. ensure its
own preferred memory layout.
