Without this, tables with width equal to a percentage that have column widths that are fit
will not have the fit columns stretch properly.
A test case is:
<math display="block">
<mtable width="100%" columnwidth="3em fit 3em">
<mtr>
<mtd mathbackground="red"><mi>x</mi></mtd>
<mtd mathbackground="yellow"><mi>x</mi></mtd>
<mtd mathbackground="red"><mi>x</mi></mtd>
</mtr>
</mtable>
</math>
Without the patch, the two outer (red) cells will be too wide. With the patch, the inner (yellow) cell will stretch to fill all the extra space.