[plt-scheme] faster text drawing in Mac OS X

DrScheme's editor has long suffered from slow text drawing under OS X,
but I've finally succeeded in making text drawing faster in MrEd --- by
a factor of 10 or so, which makes the speed comparable to other
platforms. DrScheme on OS X is now more responsive when scrolling
line-by-line or when editing a long, wrapped line.

The changes to MrEd have been exp-tagged. The solution is fairly
complex, and it uses an undocumented Quartz function for
partly-smoothed mode (which is the default for fixed-with text), so
keep an eye out for problems.

Matthew