Represents the style properties, whether from presentation attributes, the
sp_object_private_set doesn't handle SP_ATTR_STYLE or any presentation attributes at the time of writing, so this is probably NULL for all SPObject's that aren't an SPItem.
However, this gives rise to the bugs mentioned in sp_object_get_style_property. Note that some non-SPItem SPObject's, such as SPStop, do need styling information, and need to inherit properties even through other non-SPItem parents like <defs>.
Referenced by Inkscape::Text::Layout::Calculator::_computeFontLineHeight(), graphlayout(), SPItem::isExplicitlyHidden(), SPObject::releaseReferences(), SPItem::setExplicitlyHidden(), sp_object_get_style_property(), sp_object_private_set(), sp_tref_convert_to_tspan(), stroke_average_width(), and SPObject::updateDisplay().