fillRect(x1, y1, x2, y2, fillStyle) {
    this._ctx.beginPath();
    this._ctx.fillStyle = fillStyle;
    this._ctx.fillRect(x1, y1, x2, y2);
    this._ctx.closePath();
  }
Create a filled rectangle
@param {number} x1
@param {number} y1
@param {number} x2
@param {number} y2
@param {string} fillStyle