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            -1           ..FV..... keep the same input field
     bff             0            ..FV..... mark as bottom-field-first
     tff             1            ..FV..... mark as top-field-first
     prog            2            ..FV..... mark as progressive


