Apply accumulated uniform scaling to text size

This is the same change that has helped in Cairo and elsewhere.  Take the uniform scaling part of `tt`, and multiply the text size that the backend uses to calculate font metrics.  Rendering of the example in #1 is greatly improved in Firefox and Chromium.  It's not perfect, but it's at least reasonable -- in a real diagram I'd make the square 1.1 or 1.2 and call it a day.