appendBuffer(data, timestampOffset, stream) {
    const ContentType = shaka.util.ManifestParserUtils.ContentType;
    // LCEVC only supports VIDEO.
    if (stream.type !== ContentType.VIDEO ||
        (this.isDualTrack_ && !stream.baseOriginalId)) {
      return;
    }
    if (this.dec_) {
      // Timestamp offset describes how much offset should be applied to the
      // LCEVC enhancement to match the decoded video frame, as such it needs
      // to be negated.
      this.dec_.appendBuffer(data, 'video', stream.id, -timestampOffset,
          !this.isDualTrack_);
    }
  }
Append data to the LCEVC Dec.
@param {BufferSource} data
@param {number} timestampOffset
@param {shaka.extern.Stream} stream