cssinitialvalue: Make _compute public

So we can use it in cases where the class of css value is known to be an
initial one.

gtk/gtkcssinitialvalue.c

@@ -37,7 +37,7 @@ gtk_css_value_initial_free (GtkCssValue *value)
   g_assert_not_reached ();
 }
 
-static GtkCssValue *
+GtkCssValue *
 gtk_css_value_initial_compute (GtkCssValue      *value,
                                guint             property_id,
                                GtkStyleProvider *provider,

gtk/gtkcssinitialvalueprivate.h

@@ -27,6 +27,13 @@ G_BEGIN_DECLS
 GtkCssValue *   _gtk_css_initial_value_new            (void);
 GtkCssValue *   _gtk_css_initial_value_get            (void);
 
+GtkCssValue *   gtk_css_value_initial_compute         (GtkCssValue      *value,
+                                                       guint             property_id,
+                                                       GtkStyleProvider *provider,
+                                                       GtkCssStyle      *style,
+                                                       GtkCssStyle      *parent_style);
+
+
 G_END_DECLS
 
 #endif /* __GTK_CSS_INITIAL_VALUE_PRIVATE_H__ */