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.

test/read_mixer.c

@@ -19,6 +19,8 @@
 
 #include "mixer_control.h"
 #include "mixer_list.h"
+#include "filter_registry.h"
+#include "pcm_volume.h"
 
 #include <glib.h>
 
@@ -26,6 +28,22 @@
 #include <string.h>
 #include <unistd.h>
 
+const struct filter_plugin *
+filter_plugin_by_name(G_GNUC_UNUSED const char *name)
+{
+	assert(false);
+	return NULL;
+}
+
+bool
+pcm_volume(G_GNUC_UNUSED void *buffer, G_GNUC_UNUSED int length,
+	   G_GNUC_UNUSED const struct audio_format *format,
+	   G_GNUC_UNUSED int volume)
+{
+	assert(false);
+	return false;
+}
+
 int main(int argc, G_GNUC_UNUSED char **argv)
 {
 	struct mixer *mixer;