FFMPEG Plugin: Fix correct decoding of first video frame.

- The first decoded video frame was always zero bytes large, due to passing the
  wrong linesize to the color conversion function. The field containing the
  right linesize (fHeader.u.raw_video.bytes_per_row) simply wasn't initialized
  yet. Fix it by updating fHeader first before applying deinterlacing and color
  converting to the decoded video frame.
  Tested with mpeg2_decoder_test where the first picture now isn't empty
  (black) anymore.