Fix TeX output so that it matches HTML output (apply math_ev3 to it).

This fixes the problem reported in [Bugzilla 2840](http://bugs.webwork.maa.org/show_bug.cgi?id=2840) where the math displayed in HTML might not the same as that in TeX.  The problem was that HTML mode went through `math_ev3` which does some pre-processing (like changing `+-` to `-`), but TeX didn't.  This patch makes both go through `math_ev3`.