swaybar: do not retry search for tray icons

In case a tray icon cannot be found or does not have a desirable size,
swaybar retries the search again and again, which increases load on disk
and CPU. This commit solves it by storing target_size for each icon, so
that swaybar does not search for an icon of some size if it already tried to.

Fixes #3789.

include/swaybar/tray/item.h

@@ -20,6 +20,7 @@ struct swaybar_sni {
 	cairo_surface_t *icon;
 	int min_size;
 	int max_size;
+	int target_size;
 
 	// dbus properties
 	char *watcher_id;