function isElementPartiallyInViewport(el, ignoreSize) {
    var rect = el.getBoundingClientRect();
    var windowHeight = (window.innerHeight || document.documentElement.clientHeight);
    var windowWidth = (window.innerWidth || document.documentElement.clientWidth);

    return (ignoreSize || isElementDrawn(el, rect))
        && (rect.top < windowHeight) && (rect.bottom > 0)
        && (rect.left < windowWidth) && (rect.right > 0);
}
Check whether an element is in viewport.

@param {Element} el the element to be checked.
@param {boolean} [ignoreSize=false] whether to ignore size of the element, otherwise the element must be with size 4*4.
@returns {boolean}