Fitting Text To A Given Width

I have been adding some code to my Qahirah Notebooks
<https://github.com/ldo/qahirah_notebooks> demonstrating how to fit
text to a given width (in the “Beginning Text” notebook). The example
uses a list of text items of wildly differing width, so no single font
size looks best for all. Instead, I apply a non-uniform scaling to the
text to make it fit.

The core of the (Python) code looks like this:

    ctx.font_matrix = uniform_font_matrix
    text_width = ctx.text_extents(item).advance.x
    if text_width > fit_width :
        ctx.font_matrix = Matrix.scale((fit_width / text_width, 1)) * uniform_font_matrix
    #end if
    ctx.show_text(item)

However, if you try the notebook, you will see that the lines frequently
come out too wide or too narrow. I tried playing with the
HINT_STYLE_xxx and HINT_METRICS_xxx font options, but these didn’t seem
to help.

What did work was to do it in a more complex way: render the text to a
separate ImageSurface with uniform scaling, then apply nonuniform
scaling to that as a pattern to render it back to the final image. This
gives me much more consistent results.

Is there some rounding going on in the text rendering?
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