Add support for only rendering content that is visible

[brikis98]

When a pagelet arrives in the browser, check the location of its placeholder div. If that div is not currently visible (e.g. it's below the fold), don't waste CPU cycles rendering the content. Instead, attach a scroll event handler and only render the content if/when the user scrolls to it. This can dramatically reduce load time for tall pages with lots of content/images below the fold.