opt: monitor rule fallback to custom mode

src/config/parse_config.h

@@ -2625,6 +2625,10 @@ void reapply_monitor_rules(void) {
 													mr->height, mr->refresh);
 					if (internal_mode) {
 						wlr_output_state_set_mode(&state, internal_mode);
+					} else {
+						wlr_output_state_set_custom_mode(
+							&state, mr->width, mr->height,
+							(int)roundf(mr->refresh * 1000));
 					}
 				}