[racket-dev] gracket on retina

I've pushed changes to support Mac Retina displays, and the changes are
included in snapshots here:

  http://www.cs.utah.edu/plt/snapshots/

We haven't yet upgraded various bitmaps (splash screen, tool icons,
etc.) to higher-res versions, and the API needs a little more work on
that end.

But I expect that Retina users are mainly interested in text, and text
should now look right (so let me know if it doesn't).