setPixelRatio(pixelRatio) {
        if(pixelRatio !== this.pixelRatio) {
            this._shouldUpdate = true;
        }

        this.pixelRatio = pixelRatio;
    }
Sets the camera pixel ratio value
     Update the camera projection matrix only if the pixel ratio actually changed

     params:
     @pixelRatio (float, optional): pixelRatio value to use
*