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_api.h

@@ -0,0 +1,37 @@
+/* the Music Player Daemon (MPD)
+ * Copyright (C) 2003-2007 by Warren Dukes (warren.dukes@gmail.com)
+ * 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_API_H
+#define DECODER_API_H
+
+/*
+ * This is the public API which is used by decoder plugins to
+ * communicate with the mpd core.
+ *
+ */
+
+#include "inputPlugin.h"
+
+/**
+ * Opaque handle which the decoder plugin passes to the functions in
+ * this header.
+ */
+struct decoder;
+
+#endif