Update MathZoom.js with global border-box support
Update MathZoom.js with global border-box support

With a lot of sites using the global box-sizing it breaks the zoom box on large equations when the zoom goes wider than the screen.  This fix will explicitly add content-box on this element to prevent a global override.
wikimedia/MathJax,thomasjm/MathJax,jinntrance/mathjax,thomasjm/MathJax,GerHobbelt/MathJax,jinntrance/mathjax,dpvc/MathJax,thomasjm/MathJax,thomasjm/MathJax,GerHobbelt/MathJax,thomasjm/MathJax