fillTriangle(x1, y1, x2, y2, x3, y3, fillStyle) {
    this._ctx.beginPath();
    this._ctx.fillStyle = fillStyle;
    this._ctx.moveTo(x1, y1);
    this._ctx.lineTo(x2, y2);
    this._ctx.lineTo(x3, y3);
    this._ctx.fill();
    this._ctx.closePath();
  }
Create a filled triangle
@param {number} x1
@param {number} y1
@param {number} x2
@param {number} y2
@param {number} x3
@param {number} y3
@param {string} fillStyle