Solve Safari rendering issues with font-size overrides. (#780)

Some delicate surgery is required on the `vlist-s` table-cell CSS.
Problem with #768 reported by @ronkok; issue #779. Fixes:

* `font-size: 1px` (rather than `font-size: 0.05em`) solves render
  issues with user overrides of the `.katex` `font-size`.
* Also fix a width issue.

src/buildCommon.js

@@ -386,6 +386,9 @@ const makeVList = function(children, positionType, positionData, options) {
     }
 
     const vtable = makeSpan(["vlist-t"], rows);
+    if (rows.length === 2) {
+        vtable.classes.push("vlist-t2");
+    }
     vtable.height = maxPos;
     vtable.depth = -minPos;
     return vtable;