Set up mathjax_preprocessor

common/lib/xmodule/xmodule/js/src/capa/display.coffee

@@ -6,6 +6,7 @@ class @Problem
     @element_id = @el.attr('id')
     @url = @el.data('url')
     @render()
+    @mathjax_preprocessor = false
 
   $: (selector) ->
     $(selector, @el)
@@ -300,8 +301,8 @@ class @Problem
 
     if jax = MathJax.Hub.getAllJax(target)[0]
       eqn = $(element).val()
-      if window.mathjax_preprocessor
-        eqn = window.mathjax_preprocessor(eqn)
+      if @mathjax_preprocessor
+        eqn = @mathjax_preprocessor(eqn)
 
       MathJax.Hub.Queue ['Text', jax, eqn],
         [@updateMathML, jax, element]