output/shout: minimize the unpause latency

During the pause loop, manually sleep for 500ms if shout_delay()
returns a value greater than that.  Don't exhaust libshout's buffer.

src/output/shout_plugin.c

@@ -448,8 +448,15 @@ my_shout_play(void *data, const void *chunk, size_t size, GError **error)
 static bool
 my_shout_pause(void *data)
 {
+	struct shout_data *sd = (struct shout_data *)data;
 	static const char silence[1020];
 
+	if (shout_delay(sd->shout_conn) > 500) {
+		/* cap the latency for unpause */
+		g_usleep(500000);
+		return true;
+	}
+
 	return my_shout_play(data, silence, sizeof(silence), NULL);
 }