title: Create a padded, centered, scaled NSImage
body: <p>I'm putting a fixed size icon based on an arbitrarily sized image into an NSOutlineView using an NSImageCell.</p>
  
  <p>NSImageCell will scale the image down nicely, but I can't find a way to add padding around the scaled image.  The rows look ugly to my eye without padding.</p>
  
  <p>My first pass at fixing this problem was to create a new image of constant size, inset its bounds for padding, then use drawInRect:fromRect:operation:fraction: to draw a scaled down version of the original icon.</p>
  
  <p>That works great for square images.  I would've guessed there'd be a convenience method on NSImage to draw a centered, scaled-with-constant-aspect-ratio version of the image in a given rect, but I can't find it.</p>
  
  <p>Obviously I can calculate an appropriate rect with the same aspect ratio as the original image, but I'd rather not reinvent the wheel...</p>