inTransition(id) {
    if (!this.transition_) {
      return false;
    }
    return this.transitionStarts_[id] !== -1;
  }
Determine if a tile is in an alpha transition.  A tile is considered in
transition if tile.getAlpha() has not yet been called or has been called
and returned 1.
@param {string} id An id for the renderer.
@return {boolean} The tile is in transition.