libao2: try to identify some audio outputs that have persistent volume

The mplayer frontend (specifically, mixer.c) needs to know this. If the
audio output doesn't remember the volume across reinitialization, the
frontend will restore the volume settings. There is also the assumption
that the volume setting isn't global in this case (i.e. changing it
won't change the volume of other applications or annoy the user
otherwise).

None of these changes have been tested. I'm guessing that ESD and NAS do
per-connection non-persistent volume settings.

libao2/ao_dsound.c

@@ -418,6 +418,8 @@ static int init(int rate, int channels, int format, int flags)
     int res;
 	if (!InitDirectSound()) return 0;
 
+        global_ao->no_persistent_volume = true;
+
 	// ok, now create the buffers
 	WAVEFORMATEXTENSIBLE wformat;
 	DSBUFFERDESC dsbpridesc;

libao2/ao_esd.c

@@ -164,6 +164,8 @@ static int init(int rate_hz, int channels, int format, int flags)
     float lag_seconds, lag_net = 0., lag_serv;
     struct timeval proto_start, proto_end;
 
+    global_ao->no_persistent_volume = true;
+
     if (esd_fd < 0) {
 	esd_fd = esd_open_sound(server);
 	if (esd_fd < 0) {

libao2/ao_nas.c

@@ -424,6 +424,8 @@ static int init(int rate,int channels,int format,int flags)
 
 	(void)flags; /* shut up 'unused parameter' warning */
 
+	global_ao->no_persistent_volume = true;
+
 	nas_data=malloc(sizeof(struct ao_nas_data));
 	memset(nas_data, 0, sizeof(struct ao_nas_data));
 

libao2/ao_openal.c

@@ -108,6 +108,7 @@ static int init(int rate, int channels, int format, int flags) {
   const opt_t subopts[] = {
     {NULL}
   };
+  global_ao->no_persistent_volume = true;
   if (subopt_parse(ao_subdevice, subopts) != 0) {
     print_help();
     return 0;

libao2/ao_sdl.c

@@ -135,6 +135,8 @@ static int init(int rate,int channels,int format,int flags){
 	/* SDL Audio Specifications */
 	SDL_AudioSpec aspec, obtained;
 
+        global_ao->no_persistent_volume = true;
+
 	/* Allocate ring-buffer memory */
 	buffer = av_fifo_alloc(BUFFSIZE);