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

We should probably improve the contracts on `racket/draw` to promise
flonum results for text metrics. The intent is to make metric-derived
calculations have a predictable cost, instead of potentially triggering
expensive exact arithmetic.

When you say that Pango produces "exact" results, do you mean "integer"
or "exact rational"? The latter is certainly true: Pango's raw API
produces integers to be divided by 1024 to convert to drawing units.
Taking that conversion into account, Pango doesn't always produce
integer drawing units (at least on some platforms; I'm not sure about
all). Even so, the intent is that representing an integer divided by
1024 as a flonum will not normally lose any prevision (i.e., for normal
drawing sizes), and so `inexact->exact` on the immediate result from
`racket/draw` recovers the exact Pango result when exact arithmetic is
specifically wanted.

At Mon, 1 Dec 2014 11:56:12 -0800, Matthew Butterick wrote:
> 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?
> 
> 
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users