[CommonHTML] rowlines can be too short

In the CommonHTML output, a row line can end up too short when the row above is shorter than the maximal row in the table (or perhaps the next row, I'm not sure what the spec says).

This seems due to rowlines being drawn via border-bottom on the elements from the row above the rowline; that row would need additional empty elements at its end (to match the rowline length) to ensure a correct line.

For example,

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mtable data-mml-elementary="mstack" columnspacing="0" rowlines="none solid none">
    <mtr>
      <mtd />
      <mtd>
        <mn>4</mn>
      </mtd>
      <mtd>
        <mn>2</mn>
      </mtd>
      <mtd>
        <mn>4</mn>
      </mtd>
      <mtd data-mml-elementary-decimalpos="true">
        <mn>.</mn>
      </mtd>
      <mtd>
        <mn>2</mn>
      </mtd>
    </mtr>
    <mtr data-mml-elementary-msrow-op-front="true">
      <mtd>
        <mo>+</mo>
      </mtd>
      <mtd>
        <none />
      </mtd>
      <mtd>
        <mn>3</mn>
      </mtd>
      <mtd>
        <mn>3</mn>
      </mtd>
    </mtr>
    <mtr>
      <mtd />
      <mtd>
        <mn>4</mn>
      </mtd>
      <mtd>
        <mn>5</mn>
      </mtd>
      <mtd>
        <mn>7</mn>
      </mtd>
      <mtd data-mml-elementary-decimalpos="true">
        <mn>.</mn>
      </mtd>
      <mtd>
        <mn>2</mn>
      </mtd>
    </mtr>
  </mtable>
</math>