reinitText(mimeType, sequenceMode, external) {
    if (!this.textEngine_) {
      this.textEngine_ = new shaka.text.TextEngine(this.textDisplayer_);
      if (this.textEngine_) {
        this.textEngine_.setModifyCueCallback(this.config_.modifyCueCallback);
      }
    }
    this.textEngine_.initParser(mimeType, sequenceMode,
        external || this.segmentRelativeVttTiming_, this.manifestType_);
  }
Reinitialize the TextEngine for a new text type.
@param {string} mimeType
@param {boolean} sequenceMode
@param {boolean} external