getExtension(name) {
    if (name in this.extensionCache_) {
      return this.extensionCache_[name];
    }
    const extension = this.gl_.getExtension(name);
    this.extensionCache_[name] = extension;
    return extension;
  }
Get a WebGL extension.  If the extension is not supported, null is returned.
Extensions are cached after they are enabled for the first time.
@param {string} name The extension name.
@return {Object|null} The extension or null if not supported.