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.
We can't print shorthands, so don't try.
In particular, I want to get away from shorthands being representable
using GValue, and this function kinda requires that.
