decoder_api: automatically send stream tag

If an input stream provides tags (e.g. from an icecast server), send
them in the decoder_data() and decoder_tag() methods.  Removed the
according code from the mp3 and oggvorbis plugins - decoders shouldn't
have to care about stream tags.

This patch also adds the missing decoder_tag() invocation to the mp3
plugin.

src/decoder_internal.h

@@ -28,6 +28,9 @@ struct decoder {
 	struct pcm_convert_state conv_state;
 
 	bool seeking;
+
+	/** has the tag from the input stream been sent yet? */
+	bool stream_tag_sent;
 };
 
 #endif