Fix deinterlacing flags to make more sense. The TFF (top-field-first)
flag is meant to specify the organization of reference frames used in
advanced deinterlacing modes. Introduce the more explicit flag TOPFIELD
to specify that the top-field of the supplied input surface is to be
used for deinterlacing. Conversely, if not set, this means that the
bottom field of the supplied input surface will be used instead.