_getRelativeMousePosition = function(event, element, pixelRatio) {
        var elementBounds = element.getBoundingClientRect(),
            rootNode = (document.documentElement || document.body.parentNode || document.body),
            scrollX = (window.pageXOffset !== undefined) ? window.pageXOffset : rootNode.scrollLeft,
            scrollY = (window.pageYOffset !== undefined) ? window.pageYOffset : rootNode.scrollTop,
            touches = event.changedTouches,
            x, y;
        
        if (touches) {
            x = touches[0].pageX - elementBounds.left - scrollX;
            y = touches[0].pageY - elementBounds.top - scrollY;
        } else {
            x = event.pageX - elementBounds.left - scrollX;
            y = event.pageY - elementBounds.top - scrollY;
        }

        return { 
            x: x / (element.clientWidth / (element.width || element.clientWidth) * pixelRatio),
            y: y / (element.clientHeight / (element.height || element.clientHeight) * pixelRatio)
        };
    }
Gets the mouse position relative to an element given a screen pixel ratio.
@method _getRelativeMousePosition
@private
@param {} event
@param {} element
@param {number} pixelRatio
@return {}