added decoder_clear() and decoder_flush()

We are now beginning to remove direct structure accesses from the
decoder plugins.  decoder_clear() and decoder_flush() mask two very
common buffer functions.

src/decoder_api.c

@@ -109,3 +109,13 @@ int decoder_data(mpd_unused struct decoder *decoder, InputStream * inStream,
 
 	return 0;
 }
+
+void decoder_flush(mpd_unused struct decoder *decoder)
+{
+	ob_flush();
+}
+
+void decoder_clear(mpd_unused struct decoder *decoder)
+{
+	ob_clear();
+}