Fix #711 issue with multiple superscripts.

The previous code incorrectly implemented TeXbook rules. First off,
the font metrics at issue should be those for the superscript/
subscript style, not the main style. Secondly, the metrics should be
scaled by the font size.

This changes some screenshotter tests (and they look more like TeX
now).

src/buildHTML.js

@@ -334,8 +334,13 @@ groupTypes.supsub = function(group, options) {
         supShift = 0;
         subShift = 0;
     } else {
-        supShift = base.height - style.metrics.supDrop;
+        const supstyle = style.sup();
-        subShift = base.depth + style.metrics.subDrop;
+        supShift = base.height - supstyle.metrics.supDrop
+            * supstyle.sizeMultiplier;
+
+        const substyle = style.sub();
+        subShift = base.depth + substyle.metrics.subDrop
+            * substyle.sizeMultiplier;
     }
 
     // Rule 18c

src/fontMetrics.js

@@ -15,7 +15,7 @@ const cjkRegex = require("./unicodeRegexes").cjkRegex;
 // the arrays below, in that order.
 //
 // The font metrics are stored in fonts cmsy10, cmsy7, and cmsy5 respsectively.
-// This was determined by running the folllowing script:
+// This was determined by running the following script:
 //
 //     latex -interaction=nonstopmode \
 //     '\documentclass{article}\usepackage{amsmath}\begin{document}' \