css: Add a scale argument to css-value compute vfunc

We need to be able to compute different GtkCssImage values
depending on the scale, and we need this at compute time so that
we don't need to read any images other than the scale in used (to
e.g. calculate the image size). GtkStyleProviderPrivate is shared
for all style contexts, so its not right.

gtk/gtkcsslookupprivate.h

@@ -55,6 +55,7 @@ void                    _gtk_css_lookup_set_computed            (GtkCssLookup
                                                                  GtkCssValue                *value);
 void                    _gtk_css_lookup_resolve                 (GtkCssLookup               *lookup,
                                                                  GtkStyleProviderPrivate    *provider,
+								 int                         scale,
                                                                  GtkCssComputedValues       *values,
                                                                  GtkCssComputedValues       *parent_values);