getAlpha(id, time) {
    if (!this.transition_) {
      return 1;
    }

    let start = this.transitionStarts_[id];
    if (!start) {
      start = time;
      this.transitionStarts_[id] = start;
    } else if (start === -1) {
      return 1;
    }

    const delta = time - start + 1000 / 60; // avoid rendering at 0
    if (delta >= this.transition_) {
      return 1;
    }
    return easeIn(delta / this.transition_);
  }
Get the alpha value for rendering.
@param {string} id An id for the renderer.
@param {number} time The render frame time.
@return {number} A number between 0 and 1.