swaybar: don't wl_display_roundtrip on each frame

This was the source of numerous bugs, from hotplug events not being
received to segfaults because wl_display_roundtrip was making the
bar process unplug events while blocking in an iteration over all
outputs.

swaybar/bar.c

@@ -558,6 +558,7 @@ void bar_run(struct swaybar *bar) {
 	}
 	while (1) {
 		event_loop_poll();
+		wl_display_flush(bar->display);
 	}
 }