mixer/software: new mixer which controls filter/volume

This mixer plugin may be used instead of the traditional global
software mixer.  It integrates with the "volume" filter plugin, and
can control the software volume of an audio output which has no
hardware mixer.

src/mixer/software_mixer_plugin.h

@@ -0,0 +1,33 @@
+/*
+ * 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.
+ */
+
+#ifndef SOFTWARE_MIXER_PLUGIN_H
+#define SOFTWARE_MIXER_PLUGIN_H
+
+struct mixer;
+struct filter;
+
+/**
+ * Returns the (volume) filter associated with this mixer.  All users
+ * of this mixer plugin should install this filter.
+ */
+struct filter *
+software_mixer_get_filter(struct mixer *mixer);
+
+#endif