Fix bounding box computation for some embellished operators. (mathjax/MathJax#3250)

[dpvc]

This PR fixes a bounding box computation for some embellished operators when inline breaking is enabled. For example:

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mn>1</mn>
  <mrow data-mjx-texclass="REL" mathbackground="red">
    <mover>
      <mo stretchy="false">&#x27F5;</mo>
      <mi>a</mi>
    </mover>
  </mrow>
  <mn>2</mn>
</math>

or

1 \style{background-color: red}{\stackrel{a}{\longleftarrow}} 2

will show the wrong sized red rectangle in SVG output when in-line breaking is enabled. This PR fixes that.

Resolves issue mathjax/MathJax#3250.

[zorkow]

lgtm.