GN_Flr, Read character from filter

IN:
     IX = filter handle

OUT, if call successful:
     Fc = 0
     A = character read from filter
     Fz = 1, if character is converted, else unconverted, Fz = 0.

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_HAND ($08), bad filter handle
          RC_EOF ($09), filter is empty

Registers changed after return:
     ..BCDEHL/IXIY same
     AF....../.... different