+ * SET_WCB() is used as:
+ *
+ * SET_WCB( window, Visibility, func );
+ *
+ * ...where {window} is the freeglut window to set the callback,
+ * {Visibility} is the window-specific callback to set,
+ * {func} is a function-pointer.
+ *
+ * Originally, {FETCH_WCB( ... ) = func} was rather sloppily used,
+ * but this can cause warnings because the FETCH_WCB() macro type-
+ * casts its result, and a type-cast value shouldn't be an lvalue.
+ *
+ * The {if( FETCH_WCB( ... ) != func )} test is to do type-checking
+ * and for no other reason. Since it's hidden in the macro, the
+ * ugliness is felt to be rather benign.