Force an identity matrix when creating a pango layout

I've been seeing strange effects in the font rendering when zooming in
dramatically. Inter-character and inter-word spacing was becoming
radically large or radically close to zero, (with overlapping
characters, etc.).

I assume the problem has something to do with numbers outside the
representable range of Pango's fixed-width implementation, (see "pango
units"). Simply setting an identity matrix before creating the layout
and selecting the font clears up all problems that I have seen so far.