Filter field
  Extract a field from the input video.
    Inputs:
       #0: default (video)
    Outputs:
       #0: default (video)
field AVOptions:
  type              <int>        ..FV..... set field type (top or bottom) (from 0 to 1) (default top)
     top             0            ..FV..... select top field
     bottom          1            ..FV..... select bottom field


