support for `mtr` styling

Not sure if that's expected behavior but it seems like something we ought to do at some point.

The following styling of the row is lost (either which way the styling is done).

  <math xmlns="http://www.w3.org/1998/Math/MathML">
    <mtable >
      <mtr mathbackground="yellow" style="color:yellow;">
        <mtd>
          <mn>1</mn>
        </mtd>
        <mtd>
          <mn>0</mn>
        </mtd>
      </mtr>
      <mtr>
        <mtd>
          <mn>3</mn>
        </mtd>
        <mtd>
          <mn>4</mn>
        </mtd>
      </mtr>
    </mtable>
  </math>