Properly handle 'fit' columns in percentage-width tables.

[dpvc]

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.