config/output: Allow approximation of output refresh rate

Previous behavior was that only if resolution and refresh rate match
exactly, the mode was accepted. As fallback, the mode with the highest
refresh rate and the same resolution was chosen.

New behavior is that the mode with the closest match for the refresh
rate is used with a limit of up to 1Hz. The fallback behavior stays the same.

Additionally, the logging was made more verbose.

sway/config/output.c

@@ -249,6 +249,8 @@ static void set_mode(struct wlr_output *output, struct wlr_output_state *pending
 	// as (int)(1000 * mHz / 1000.f)
 	// round() the result to avoid any error
 	int mhz = (int)roundf(refresh_rate * 1000);
+	// If no target refresh rate is given, match highest available
+	mhz = mhz <= 0 ? INT_MAX : mhz;
 
 	if (wl_list_empty(&output->modes) || custom) {
 		sway_log(SWAY_DEBUG, "Assigning custom mode to %s", output->name);
@@ -258,23 +260,28 @@ static void set_mode(struct wlr_output *output, struct wlr_output_state *pending
 	}
 
 	struct wlr_output_mode *mode, *best = NULL;
+	int best_diff_mhz = INT_MAX;
 	wl_list_for_each(mode, &output->modes, link) {
 		if (mode->width == width && mode->height == height) {
-			if (mode->refresh == mhz) {
+			int diff_mhz = abs(mode->refresh - mhz);
+			if (diff_mhz < best_diff_mhz) {
+				best_diff_mhz = diff_mhz;
 				best = mode;
+				if (best_diff_mhz == 0) {
 					break;
 				}
-			if (best == NULL || mode->refresh > best->refresh) {
-				best = mode;
 			}
 		}
 	}
-	if (!best) {
-		sway_log(SWAY_ERROR, "Configured mode for %s not available", output->name);
-		sway_log(SWAY_INFO, "Picking preferred mode instead");
-		best = wlr_output_preferred_mode(output);
+	if (best) {
+		sway_log(SWAY_INFO, "Assigning configured mode (%dx%d@%.3fHz) to %s",
+			best->width, best->height, best->refresh / 1000.f, output->name);
 	} else {
-		sway_log(SWAY_DEBUG, "Assigning configured mode to %s", output->name);
+		best = wlr_output_preferred_mode(output);
+		sway_log(SWAY_INFO, "Configured mode (%dx%d@%.3fHz) not available, "
+			"applying preferred mode (%dx%d@%.3fHz)",
+			width, height, refresh_rate,
+			best->width, best->height, best->refresh / 1000.f);
 	}
 	wlr_output_state_set_mode(pending, best);
 }