cssnode: Move clear_classes() into the NodeDeclaration

The previous code was crashing when used as the returned classes array
would have been invalid after the first deletion. So if a 2nd class
would be deleted, invalid memory might have been referenced.

gtk/gtkcssnodedeclaration.c

@@ -307,6 +307,21 @@ gtk_css_node_declaration_remove_class (GtkCssNodeDeclaration **decl,
   return TRUE;
 }
 
+gboolean
+gtk_css_node_declaration_clear_classes (GtkCssNodeDeclaration **decl)
+{
+  if ((*decl)->n_classes == 0)
+    return FALSE;
+
+  gtk_css_node_declaration_make_writable_resize (decl,
+                                                 (char *) get_classes (*decl) - (char *) *decl,
+                                                 0,
+                                                 sizeof (GQuark) * (*decl)->n_classes);
+  (*decl)->n_classes = 0;
+
+  return TRUE;
+}
+
 gboolean
 gtk_css_node_declaration_has_class (const GtkCssNodeDeclaration *decl,
                                     GQuark                       class_quark)