Make all the lines the correct sizes

Fix the \frac and \overline lines so they actually render at the correct sizes
(i.e. don't get smaller when in script and scriptscript sizes). This also seems
to fix #33.

Test Plan:
 - Make sure new huxley screenshot look good.
 - Render
     \frac{1}{\Bigl(\sqrt{\phi \sqrt{5}}-\phi\Bigr) e^{\frac25 \pi}} =
     1+\frac{e^{-2\pi}} {1+\frac{e^{-4\pi}} {1+\frac{e^{-6\pi}}
     {1+\frac{e^{-8\pi}} {1+...} } } }
   at small font sizes like 30pt and smaller, see that it looks much better

Auditors: alpert