Each filter using the common deinterlacing code now calls DoDrain (similar to
DoDeinterlacing) to do its draining.

We no longer return pictures chained using vlc_picture_chain_AppendChain().

DoDraining calls DoDeinterlacing internally with a NULL input picture. To
create secondary pictures, the state to create the same pictures that were
appended before is kept in the filter p_sys. The prev_pic is the original
picture used to create the primary and secondary pictures. The picture is
released when there is no more pictures to produce from this picture or during
a Flush.

Now DoDeinterlacing() only outputs one picture at a time. By default it outputs
the current_field == 0 picture, then extra pictures up to max_fields.