Use superscript for scriptChild in msubsup. (mathjax/MathJax#3097)

[dpvc]

This PR fixes a problem with the placement of msubsup scripts when when the depth of either script is large. The problem was due to the getU() method using this.scriptChild, where sciptChild was always set to childNodes[1]; for msubsup nodes, it should be childNodes[2], the superscript. The solution is to override scriptChild in the CommonMsubsupMixin wrapper. (I also cached the superscript depth and subscript height values that are used in several places).

The issue would show up if the superscript has large depth, as in

$$\int_0^{\Rule{3pt}{0pt}{20pt}}$$

which produces

or if the subscript has large depth, as in

$$x_{\Rule{3pt}{0pt}{20pt}}^2$$

which produces

With this PR, both produce the proper results:

Resolves issue mathjax/MathJax#3097.

[zorkow]

lgtm.