If we depend on whatever Fontconfig finds on the system (or the user's
configuration, or any number of variables not under our control), "make
distcheck" will certainly fail.

Now we ship a specific font file (LiberationSans-Regular.ttf) that is
used only for the reftests.  We use that in addition to explicitly
setting font rendering options for tests.

Also, re-normalize all the reference images to this new font.

Thanks to Vlad Vukicevic for the idea to use a hard-coded font file, and
to Christian Hergert for the Fontconfig / Pango incantations.