Allow columns to shrink during line breaking. (mathjax/MathJax#3235)

[dpvc]

During line breaking, when a table column has breaks, the computed width was not always being reduced, leaving the column width the original size, leading to the output bing too wide despite the line breaking.

This PR resolves the problem by allowing columns that are fit, auto, or percentage-width to shrink to the width of the newly broken cells rather than their original widths.

A test case is

<div style="width:20em; border:1px solid black">
\begin{equation*}
\begin{cases}
\def\div{\operatorname{div}}-\div(A(x)Du)=c_\lambda(x)u+( M(x)Du,Du)+h(x)\\
u\in H_0^1(\Omega)\cup L^\infty(\Omega)
\end{cases}
\tag{$P_\lambda$}\label{$P_lambda$}
\end{equation*}
</div>

with overflow set to linebreaks and displayAlign set to left. Without this PR, the tag will be outside the div, and with it, the tag should be properly placed.

Resolves issue mathjax/MathJax#3235.

[zorkow]

Works for me.