If we use \newcommand in a TeX math, the influences will be persistent in current process.
The expected result is -- all \newcommands should only affect the current requested page.
The harm is that if I run a randering server with this library, a malicious person can submit a request that replaces builtin commands with their owns with \renewcommand, so later requests won't return correct result.
This upstream PR may help: mathjax/MathJax-node#348.