We need to store the border widths independant of them being set to 0 by
border styles, because otherwise we'd need to track that dependency and
recompute on changes, and I don't want to add more entries to
GtkCssDependencies just for this special case.

By moving the code that does the setting to 0 from the compute stage to
the query stage, we can achieve this.

Now we need to just be aware that the actual value stored is not set to
0 when we use gtk_css_computed_values_get_value().