... so we don't bump a refcount whenever we get the initial singleton. We want to use this function instead of _gtk_css_style_property_get_initial_value() everywhere where we compute values, because some initial values may depend on settings soon.
... to the compute vfunc. Simplifies code quite a bit. But makes the code no longer a simple step-by-step implementation of the spec.
Also split out initial/inherit handling into a custom GtkCssValue class.