encoder_plugin: add method pre_tag()

In the "vorbis" plugin, this is a copy of the old flush() method,
while flush() gets a lot of code remove, it just sets the "flush" flag
and nothing else.  It doesn't start a new stream now, which should fix
a few problems in some players.

src/output/httpd_output_plugin.c

@@ -523,7 +523,7 @@ httpd_output_tag(void *data, const struct tag *tag)
 
 		/* flush the current stream, and end it */
 
-		encoder_flush(httpd->encoder, NULL);
+		encoder_pre_tag(httpd->encoder, NULL);
 		httpd_output_encoder_to_clients(httpd);
 
 		/* send the tag to the encoder - which starts a new

src/output/shout_plugin.c

@@ -511,7 +511,7 @@ static void my_shout_set_tag(void *data,
 	if (sd->encoder->plugin->tag != NULL) {
 		/* encoder plugin supports stream tags */
 
-		ret = encoder_flush(sd->encoder, &error);
+		ret = encoder_pre_tag(sd->encoder, &error);
 		if (!ret) {
 			g_warning("%s", error->message);
 			g_error_free(error);