added struct decoder

The decoder struct should later be made opaque to the decoder plugin,
because maintaining a stable struct ABI is quite difficult.  The ABI
should only consist of a small number of stable functions.

src/decoder_internal.h

@@ -0,0 +1,28 @@
+/* the Music Player Daemon (MPD)
+ * Copyright (C) 2008 Max Kellermann <max@duempel.org>
+ * This project's homepage is: http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifndef DECODER_INTERNAL_H
+#define DECODER_INTERNAL_H
+
+#include "decoder_api.h"
+
+struct decoder {
+	InputPlugin *plugin;
+};
+
+#endif