* cigaes/master:
  lavf/matroskaenc: return an error for unsupported types.
  lavf/concatdec: remove invalid check for AVSEEK_FLAG_BACKWARD.
  lavf: filter out AVSEEK_FLAG_BACKWARD in new API.
  lavf: call the new seek API from the old.
Merged-by: Michael Niedermayer <michaelni@gmx.at>