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

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?