[racket] how to get exact results from get-text-extent

From: Matthew Butterick (mb at mbtype.com)
Date: Mon Dec 1 14:56:12 EST 2014

The `get-text-extent` method in racket/draw does not contractually guarantee either exact or inexact numbers, though in practice I find it produces inexact. 

This function, however, calls into the Pango text-layout system. I find that when I invoke Pango's text measuring directly through the FFI, it produces exact results.

Is this difference in behavior deliberate, or does `get-text-extent` preserve exactness under certain circumstances?

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