Zoom the canvas with a relative ratio
@param {number} ratio - The target ratio.
@param {Event} _originalEvent - The original event if any.
@returns {Cropper} this
function zoom(ratio, _originalEvent) {
    var canvasData = this.canvasData;
    ratio = Number(ratio);

    if (ratio < 0) {
      ratio = 1 / (1 - ratio);
    } else {
      ratio = 1 + ratio;
    }

    return this.zoomTo(canvasData.width * ratio / canvasData.naturalWidth, null, _originalEvent);
  }