Fix a weird bug with rlap

For some reason, when you have a nested elements that look like
  `display: inline-block; > position: relative; > position: absolute;`
then the `position: absolute;` element is shifted down a bunch. If there is
anything* else inside either of the other two elements, then this behavior
disappears. (This can be seen at [this fiddle](http://jsfiddle.net/qZXRr/). We
have this structure when we create `\llap` and `\rlap`s, and this weird behavior
So, to fix this I added an empty `display: inline-block;` span inside the llap
to fix this.

Test plan:
 - See that the new huxley image looks good
 - Test this in a bunch of browsers and see they also look good

Auditors: alpert

static/katex.less

@@ -275,16 +275,20 @@ big parens
         width: 0;
         position: relative;
 
-        > span {
+        > .inner {
             position: absolute;
         }
+
+        > .fix {
+            display: inline-block;
+        }
     }
 
-    .llap > span {
+    .llap > .inner {
         right: 0;
     }
 
-    .rlap > span {
+    .rlap > .inner {
         left: 0;
     }