This PR fixes a problem with determining the width of text that is in a system font rather than in the MathJax fonts. These come from \text{} then mtextInheritFont is true, or from error messages (when merrorInheritFont is true, which it is by default), or from characters that aren't in the MathJax fonts, like Chinese characters, or when a font-family is specified in a \style{} command or MathML style attribute.

You can't test this in the lab, as the issue only occurs when the math is typeset outside the DOM via MathJax.tex2chtml() or one of the other conversion functions, and then inserted into a container with a font-size that is other than 100%.