Draw the Slider onto its canvas context using the current .__value property.
@memberof Slider
@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;

    if (this.style === 'horizontal') this.ctx.fillRect(0, 0, this.rect.width * this.__value, this.rect.height);else this.ctx.fillRect(0, this.rect.height - this.__value * this.rect.height, this.rect.width, this.rect.height * this.__value);

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