Gets the appearance of a field (i.e. the value of
the /AP key) and stores it in a file.
fdf_documentThe FDF document handle, returned by fdf_create(), fdf_open() or fdf_open_string().
field
faceThe possible values are FDFNormalAP, FDFRolloverAP and FDFDownAP.
filenameThe appearance will be stored in this parameter.