config/output: correct refresh rate rounding error

sway/config/output.c

@@ -237,7 +237,10 @@ struct output_config *store_output_config(struct output_config *oc) {
 
 static void set_mode(struct wlr_output *output, int width, int height,
 		float refresh_rate, bool custom) {
-	int mhz = (int)(refresh_rate * 1000);
+	// Not all floating point integers can be represented exactly
+	// as (int)(1000 * mHz / 1000.f)
+	// round() the result to avoid any error
+	int mhz = (int)round(refresh_rate * 1000);
 
 	if (wl_list_empty(&output->modes) || custom) {
 		sway_log(SWAY_DEBUG, "Assigning custom mode to %s", output->name);