[sound] Add a threaded field to snd_t

Care needs to be taken when freeing channels as doing so while an
asynchronous mixer is using them is unlikely to end well. However,
whether the mixer is asynchronous depends on the output driver. This
lets the driver inform the rest of the system that the output and mixer
are running asynchronously.

include/snd_internal.h

@@ -106,6 +106,7 @@ struct snd_s {
 	int				submission_chunk;		//!< don't mix less than this #
 	unsigned        paintedtime;			//!< sound clock in samples
 	int				framepos;				//!< position of dma cursor
+	int         threaded;					//!< output runs in a thread
 	unsigned char	*buffer;				//!< destination for mixed sound
 	/** Transfer mixed samples to the output.
 		\param paintbuffer The buffer of mixed samples to be transferred.

libs/audio/renderer/snd_dma.c

@@ -186,7 +186,7 @@ static void
 s_stop_all_sounds (void)
 {
 	SND_StopAllSounds (&snd);
-	SND_ScanChannels (&snd, 0);
+	SND_ScanChannels (&snd, snd.threaded);
 	s_clear_buffer (&snd);
 }
 

libs/audio/targets/snd_alsa.c

@@ -655,6 +655,8 @@ SNDDMA_Init (snd_t *snd)
 
 	snd->frames = buffer_size;
 
+	snd->threaded = 1;//XXX FIXME double check whether it's always true
+
 	// send the first period to fill the buffer
 	// also sets snd->buffer
 	if (alsa_process (pcm, snd) < 0) {

libs/audio/targets/snd_jack.c

@@ -278,6 +278,7 @@ static int
 s_init (snd_t *snd)
 {
 	snd->xfer = snd_jack_xfer;
+	snd->threaded = 1;
 	return s_jack_connect (snd);
 }