Properly invalidate rendering cache when updating orientation (Thanks hercek)

fixes #1445

include/con.h

@@ -356,3 +356,9 @@ void con_set_urgency(Con *con, bool urgent);
  *
  */
 char *con_get_tree_representation(Con *con);
+
+/**
+ * force parent split containers to be redrawn
+ *
+ */
+void con_force_split_parents_redraw(Con *con);