[mml3.js] fails in QT QWebpage environments on Windows

Via https://github.com/pkra/MathJax-single-file/issues/3

It seems mml3 silently fails to pre-process, passing the input along.

It could be a browser bug (perhaps xslt support is stripped). But the issue belongs here and I want to look into it at some point (or fix it by re-writing mml3).

Note that QT has similarly sounding parts (QT WebEngine) that uses Chromium.