Change frac-line from border to full span (#976)

* Change frac-line from border to full span

Change `frac-line`  and `vertical-separator`.  Instead of using span borders, fill entire span using a `box-shadow`.

This change will enable more dependable engagement of the `min-height` CSS.

* Fix indentation

* Fix indent again

* Change box shadow to inline SVG

* Fix lint error

* regenerate screenshots after switching to SVG for fraction bars

src/svgGeometry.js

@@ -5,6 +5,10 @@
  */
 
 const path: {[string]: string} = {
+    // bigRule provides a big square image for frac-lines, etc.
+    // The actual rendered rule is smaller, controlled by overflow:hidden.
+    bigRule: 'M0 0 h400000 v400000 h-400000z M0 0 h400000 v400000 h-400000z',
+
     // sqrtMain path geometry is from glyph U221A in the font KaTeX Main
     sqrtMain: `M95 622c-2.667 0-7.167-2.667-13.5
 -8S72 604 72 600c0-2 .333-3.333 1-4 1.333-2.667 23.833-20.667 67.5-54s