Also, in places where we're computing a new CssValue based on an
old one, make sure that if nothing changes we're returning a reference
to the old one, rather than creating a new identical instance.
Instead of on-demand resolvage, we now resolve during lookup. The step
is done via
_gtk_css_style_property_compute_value()
which currently calls into
_gtk_css_style_compute_value()
That function has all the old resolving machinery.
The only part missing for now is the handling of win32 code. It will be
added back later.
