Correct computation of TeX sizes. (#755)

* Em and mu sizes are relative to current font's quad metric,
  which might not be "1em" in CSS terms.
* We don't need the `emPerEx` metric, because we want to convert
  TeX `ex` to CSS `em`, not TeX `ex` to TeX `em`. The `xHeight`
  metric does that already: it is relative to font size.
* Mu sizes scale with the style, em and ex do not (empirically
  checked; agrees with rule 2 of TeXbook Appendix G).
  This corrects a bug in b866cd5224 (in which em scaled),
  and a bug in previous versions (in which mu did not scale).
* Correct rule sizes. Previous comment---"the sizes of rules are
  absolute"---was misleading. Rule sizes are NOT absolute---in
  \large size, a rule denominated in 'em' is larger. But the 'em'
  unit is not sensitive to styles. So now a 1em rule will be the
  same size in super/subscript, as it should be, but an 18mu rule
  will change size in super/subscript, as it should.

Note a TODO.

test/screenshotter/ss_data.yaml

@@ -170,6 +170,15 @@ OverUnderset: |
 Phantom: \dfrac{1+\phantom{x^{\blue{2}}} = x}{1+x^{\blue{2}} = x}
 PrimeSpacing: f'+f_2'+f^{f'}
 PrimeSuper: x'^2+x'''^2+x'^2_3+x_3'^2
+RelativeUnits: |
+    \begin{array}{ll}
+        a\kern1emb^{a\kern1emb^{a\kern1emb}} &
+        {\footnotesize a\kern1emb^{a\kern1emb^{a\kern1emb}}} \\
+        a\mkern18mub^{a\mkern18mub^{a\mkern18mub}} &
+        {\footnotesize a\mkern18mub^{a\mkern18mub^{a\mkern18mub}}} \\
+        \rule{1em}{1em}^{\rule{1em}{1em}}\rule{18mu}{18mu}^{\rule{18mu}{18mu}} &
+        {\footnotesize\rule{1em}{1em}^{\rule{1em}{1em}}\rule{18mu}{18mu}^{\rule{18mu}{18mu}}}
+    \end{array}
 RlapBug: \frac{\rlap{x}}{2}
 Rule: \rule{1em}{0.5em}\rule{1ex}{2ex}\rule{1em}{1ex}\rule{1em}{0.431ex}
 SizingBaseline: