svg_switch: implement get_extents.
svg_switch: implement get_extents.

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