getImageTracks() {
    const StreamUtils = shaka.util.StreamUtils;
    let imageStreams = this.externalSrcEqualsThumbnailsStreams_;
    if (this.manifest_) {
      imageStreams = this.manifest_.imageStreams;
    }
    return imageStreams.map((image) => StreamUtils.imageStreamToTrack(image));
  }
Return a list of image tracks that can be switched to.

If the player has not loaded content, this will return an empty list.

@return {!Array<shaka.extern.ImageTrack>}
@export