mathml: fix update of itex element.
mathml: fix update of itex element.

An update request on the itex element now forces an update of the
embededd math element, as the style may have changed.

gjtorikian/lasem,janmarthedal/mathml2svg,gjtorikian/lasem,janmarthedal/mathml2svg,wavewave/lasem,gjtorikian/lasem,wavewave/lasem,gjtorikian/lasem,wavewave/lasem,wavewave/lasem,janmarthedal/mathml2svg,janmarthedal/mathml2svg