Introducing mixer api

This patch tryes to introduce pluggable mixer (struct mixer_plugin) along with some basic infrastructure (mixer_* functions). Instance of mixer (struct mixer) is used in
alsa and oss output plugin

src/mixer_api.c

@@ -0,0 +1,45 @@
+
+#include <stdio.h>
+#include <assert.h>
+
+#include "mixer_api.h"
+
+void mixer_init(struct mixer *mixer, struct mixer_plugin *plugin)
+{
+	assert(plugin != NULL);
+	assert(mixer != NULL);
+	mixer->plugin = plugin;
+	mixer->data = mixer->plugin->init();
+}
+
+void mixer_finish(struct mixer *mixer)
+{
+	assert(mixer != NULL && mixer->plugin != NULL);
+	mixer->plugin->finish(mixer->data);
+	mixer->data = NULL;
+	mixer->plugin = NULL;
+}
+
+void mixer_configure(struct mixer *mixer, ConfigParam *param)
+{
+	assert(mixer != NULL && mixer->plugin != NULL);
+	mixer->plugin->configure(mixer->data, param);
+}
+
+bool mixer_open(struct mixer *mixer)
+{
+	assert(mixer != NULL && mixer->plugin != NULL);
+	return mixer->plugin->open(mixer->data);
+}
+
+bool mixer_control(struct mixer *mixer, int cmd, void *arg)
+{
+	assert(mixer != NULL && mixer->plugin != NULL);
+	return mixer->plugin->control(mixer->data, cmd, arg);
+}
+
+void mixer_close(struct mixer *mixer)
+{
+	assert(mixer != NULL && mixer->plugin != NULL);
+	mixer->plugin->close(mixer->data);
+}