mglyph_valignmiddle

align an mglyph at the middle.

W3C sample	WIRIS
	$\begin{matrix} \int \end{matrix}$

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mtable displaystyle="true">
    <mtr rowalign="center">
      <mtd>
        <mi>
          <mglyph src="pencil-e.png" alt="pencil to center of"/>
        </mi>
      </mtd>
      <mtd>
        <mo>&#x222B;</mo>
      </mtd>
    </mtr>
  </mtable>
</math>

Author: Bruce Miller