filter: added new plugin API for filters

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.

src/filter_internal.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2003-2009 The Music Player Daemon Project
+ * 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.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/** \file
+ *
+ * Internal stuff for the filter core and filter plugins.
+ */
+
+#ifndef MPD_FILTER_INTERNAL_H
+#define MPD_FILTER_INTERNAL_H
+
+struct filter {
+	const struct filter_plugin *plugin;
+};
+
+static inline void
+filter_init(struct filter *filter, const struct filter_plugin *plugin)
+{
+	filter->plugin = plugin;
+}
+
+#endif