_getCenterPoint() {
    const { left, top, width, height } = this.zoomArea;
    const { x, y } = this._startPoint;
    const { aspectRatio } = this;

    if (width < MOUSE_MOVE_THRESHOLD && height < MOUSE_MOVE_THRESHOLD) {
      return { x, y };
    }

    return width > height
      ? { x: left + (aspectRatio * height) / 2, y: top + height / 2 }
      : { x: left + width / 2, y: top + width / aspectRatio / 2 };
  }
Get center point
@returns {{x: number, y: number}}
@private