css: Move scale to GtkStyleProviderPrivate

This way, we can remove it as a separate argument from
gtk_css_value_compute() and allow computation to only depend on one
thing: the style provider.

gtk/gtkcssstringvalue.c

@@ -37,9 +37,8 @@ static GtkCssValue *
 gtk_css_value_string_compute (GtkCssValue             *value,
                               guint                    property_id,
                               GtkStyleProviderPrivate *provider,
-			      int                      scale,
-                              GtkCssStyle    *values,
-                              GtkCssStyle    *parent_values,
+                              GtkCssStyle             *style,
+                              GtkCssStyle             *parent_style,
                               GtkCssDependencies      *dependencies)
 {
   return _gtk_css_value_ref (value);