Draw the Joystick onto its canvas context using the current .__value property.
@memberof Joystick
@instance
function draw() {
    // draw background
    this.ctx.fillStyle = this.background;
    this.ctx.strokeStyle = this.stroke;
    this.ctx.lineWidth = this.lineWidth;
    this.ctx.fillRect(0, 0, this.rect.width, this.rect.height);

    // draw fill (slider value representation)
    this.ctx.fillStyle = this.fill;

    this.ctx.beginPath();
    this.ctx.moveTo(this.rect.width * 0.5, this.rect.height * .5);
    this.ctx.lineTo(this.rect.width * this.__value[0], this.rect.height * this.__value[1]);
    this.ctx.stroke();
    this.ctx.fillRect(this.rect.width * this.__value[0] - 12, this.rect.height * this.__value[1] - 12, 24, 24);

    this.ctx.strokeRect(0, 0, this.rect.width, this.rect.height);
  }