Clear the stylesheet when used in a new MathDocument. (mathjax/MathJax#2678)

[dpvc]

This PR makes sure the output jax clear its internal stylesheet pointer when it is used in a new document.

Resolves issue mathjax/MathJax#2678.

[zorkow]

I note there is a another this.fontCache.clearCache(); in line 300. Would it make sense to let this use the clearFontCache method?

[dpvc]

Would it make sense to let this use the clearFontCache method?

Sure. The reason I didn't was that that function was doing a lot of direct calls to this.fontCache already, so I considered the clearCache() call to be just one of those that manipulates the internal cache structure. The clearFontCache() method is more for objects outside the SVG jax to call (so they don't have to handle the internal fontCache value).

I can change it if you want, but I don't really see the need.