Set maxFontSize on rules. (#744)

This is needed in vlists, as in #646, to make fontsize-ensurer
nodes big enough.

src/buildHTML.js

@@ -1330,6 +1330,10 @@ groupTypes.rule = function(group, options) {
     rule.width = width;
     rule.height = height + shift;
     rule.depth = -shift;
+    // Font size is the number large enough that the browser will
+    // reserve at least `absHeight` space above the baseline.
+    // The 1.125 factor was empirically determined
+    rule.maxFontSize = height * 1.125 * options.sizeMultiplier;
 
     return rule;
 };

test/screenshotter/ss_data.yaml

@@ -102,6 +102,7 @@ Kern:
     tex: \frac{a\kern{1em}b}{c}a\kern{1em}b\kern{1ex}c\kern{-0.25em}d
     nolatex: LaTeX fails to typeset this, “Missing number, treated as zero.”
 Lap: ab\llap{f}cd\rlap{g}h
+LargeRuleNumerator: \frac{\textcolor{blue}{\rule{1em}{2em}}}{x}
 LeftRight: \left( x^2 \right) \left\{ x^{x^{x^{x^x}}} \right.
 LeftRightListStyling: a+\left(x+y\right)-x
 LeftRightMiddle: \left( x^2 \middle/ \right) \left\{ x^{x^{x^{x^x}}} \middle/ y \right.\left(x\middle|y\,\middle|\,z\right)