_growBottom(width, height) {
    this.root = {
      used: true,
      x: 0,
      y: 0,
      width: this.root.width,
      height: this.root.height + height,
      down: {
        x: 0,
        y: this.root.height,
        width: this.root.width,
        height
      },
      right: this.root
    };
    const node = this._findNode(this.root, width, height);

    return node ? this._splitNode(node, width, height) : null;
  }
Grow the sprite to the bottom

@param {number} width        Width
@param {number} height       Height
@returns {object|null}       Node