Generates a value between 0-1 given the current pointer position in relation
to the Slider's position, and triggers output.
@instance
@memberof Slider
@param {PointerEvent} e - The pointer event to be processed.
function processPointerPosition(e) {
    var prevValue = this.value;

    if (this.style === 'horizontal') {
      this.__value = (e.clientX - this.rect.left) / this.rect.width;
    } else {
      this.__value = 1 - (e.clientY - this.rect.top) / this.rect.height;
    }

    // clamp __value, which is only used internally
    if (this.__value > 1) this.__value = 1;
    if (this.__value < 0) this.__value = 0;

    var shouldDraw = this.output();

    if (shouldDraw) this.draw();
  }