cssvalue: Add _gtk_css_initial_value_get()

... so we don't bump a refcount whenever we get the initial singleton.

We want to use this function instead of
_gtk_css_style_property_get_initial_value() everywhere where we compute
values, because some initial values may depend on settings soon.

gtk/gtkcssinitialvalue.c

@@ -86,3 +86,9 @@ _gtk_css_initial_value_new (void)
 {
   return _gtk_css_value_ref (&initial);
 }
+
+GtkCssValue *
+_gtk_css_initial_value_get (void)
+{
+  return &initial;
+}

gtk/gtkcssinitialvalueprivate.h

@@ -25,6 +25,7 @@
 G_BEGIN_DECLS
 
 GtkCssValue *   _gtk_css_initial_value_new            (void);
+GtkCssValue *   _gtk_css_initial_value_get            (void);
 
 G_END_DECLS