deinterlace: implement draining of extra pictures
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.