title: NSCollectionView lazy-loading like behavior
body: <p>I am trying to get lazy-loading like behavior from my NSCollectionView.  (honestly I assumed it had it, like a UITableView - seems like an obvious omission?)</p>
  
  <p>Anyway, I am displaying images in the cells, and would like to load images as they are about to scroll into view, and unload them after they leave.</p>
  
  <p>This is for a research tool and does not need to be pretty - ie: stuff can "pop" in.</p>
  
  <p>Any ideas?  My first thought is to find some way for the Views in the collection to know when they're onscreen, but drawrect seems to be called many many times, far more times than there are views on screen.</p>