Filter setfield
  Force field for the output video frame.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
setfield AVOptions:
  mode              <int>        ..FV.... select interlace mode (from -1 to 2) (default auto)
     auto                         ..FV.... keep the same input field
     bff                          ..FV.... mark as bottom-field-first
     tff                          ..FV.... mark as top-field-first
     prog                         ..FV.... mark as progressive


