Reduce requestAnimationFrame polling

[asakusuma]

• Add a short gap (< 50ms) between requestAnimationFrame polls
• Only polyfill IntersectionObserver when necessary
• Via a host interface, provide a higher-order function that is passed a function that gets called anytime the DOM is updated. This guarantees that viewport status changes must occur either after this function call, on scroll, or on resize. Thus we don't have to poll and can use passive listeners on scroll and resize.