Upgrade \sqrt zoom and width (#890)

* Fix \sqrt zoom in Safari

This PR evades a Safari bug which causes nested SVGs to zoom improperly. `\sqrt` and single-ended arrow SVGs have been modified.

These have been converted from nested SVGs to single level. Their long tails are now sliced off using CSS `overflow: hidden`.

Safari will still improperly zoom any double-ended stretchy arrows and horizontal braces.

* Fix \sqrt{}

Even if the function argument is empty, still render an SVG whose width equals the surd glyph.

* Fix tall \sqrt when scaled

* update screenshots affected by sqrt fixes

* more screenshots after changes to fix sqrt

* Pick up review comments

static/katex.less

@@ -575,6 +575,13 @@
         }
     }
 
+    // Hide the long tail of a stretchy SVG.
+    .hide-tail {
+        width: 100%;          // necessary only to get IE to work properly
+        position: relative;   // ditto
+        overflow: hidden;     // This line applies to all browsers.
+    }
+
     // Lengthen the extensible arrows via padding.
     .x-arrow-pad {
         padding: 0 0.5em;