loadTexture(textureIndex) {
    const json = this.json;
    const options = this.options;
    const textureDef = json.textures[textureIndex];
    const sourceIndex = textureDef.source;
    const sourceDef = json.images[sourceIndex];
    let loader = this.textureLoader;
    if (sourceDef.uri) {
      const handler = options.manager.getHandler(sourceDef.uri);
      if (handler !== null) loader = handler;
    }
    return this.loadTextureImage(textureIndex, sourceIndex, loader);
  }
Specification: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#textures
@param {number} textureIndex
@return {Promise<THREE.Texture|null>}