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/filter/volume_filter_plugin.c

@@ -25,6 +25,7 @@
 #include "pcm_buffer.h"
 #include "pcm_volume.h"
 #include "audio_format.h"
+#include "player_control.h"
 
 #include <assert.h>
 #include <string.h>