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

test/huxley/Huxleyfile

@@ -39,3 +39,6 @@ url=http://localhost:7936/test/huxley/test.html?m=\frac{a}{b}\text{c~ {ab} \ e}+
 
 [KaTeX]
 url=http://localhost:7936/test/huxley/test.html?m=\KaTeX
+
+[RlapBug]
+url=http://localhost:7936/test/huxley/test.html?m=\rlap{x}

test/huxley/RlapBug.huxley/record.json

@@ -0,0 +1 @@
+{"py/object": "huxley.run.Test", "screen_size": {"py/tuple": [1024, 768]}, "steps": [{"py/object": "huxley.steps.ScreenshotTestStep", "index": 0, "offset_time": 0}]}