ao_alsa: do not call snd_pcm_delay() when paused

This causes trouble when a hw device is used:

    pcm_hw.c:514:(snd_pcm_hw_delay) SNDRV_PCM_IOCTL_DELAY failed (-77): File descriptor in bad state

when running mpv test.mkv --ao=alsa:device=iec958,alsa and pausing
during playback.

Historically, mplayer usually did not call snd_pcm_delay() (which is
called by get_delay()) while paused, so this problem never showed up.
But at least mpv has changes that cause get_delay() to be called when
updating the status line (see commit 3f949cf).

It's possible that calling snd_pcm_delay() is not always legal when the
audio is paused, and at least fails with the error message mentioned
above is the device is a hardware device. Change get_delay() to return
the last delay before the audio was paused. The intention is to get a
continuous playback status display, even when pausing or frame stepping,
otherwise we could just return the audio buffer fill status in
get_delay() or even just 0 when paused.

audio/out/ao_alsa.c

@@ -69,6 +69,7 @@ static size_t bytes_per_sample;
 
 static int alsa_can_pause;
 static snd_pcm_sframes_t prepause_frames;
+static float delay_before_pause;
 
 #define ALSA_DEVICE_SIZE 256
 
@@ -354,6 +355,7 @@ static int init(int rate_hz, int channels, int format, int flags)
     mp_msg(MSGT_AO,MSGL_V,"alsa-init: using ALSA %s\n", snd_asoundlib_version());
 
     prepause_frames = 0;
+    delay_before_pause = 0;
 
     snd_lib_error_set_handler(alsa_error_handler);
 
@@ -703,6 +705,7 @@ static void audio_pause(void)
     int err;
 
     if (alsa_can_pause) {
+        delay_before_pause = get_delay();
         if ((err = snd_pcm_pause(alsa_handler, 1)) < 0)
         {
             mp_tmsg(MSGT_AO,MSGL_ERR,"[AO_ALSA] pcm pause error: %s\n", snd_strerror(err));
@@ -713,6 +716,7 @@ static void audio_pause(void)
         if (snd_pcm_delay(alsa_handler, &prepause_frames) < 0
             || prepause_frames < 0)
             prepause_frames = 0;
+        delay_before_pause = prepause_frames / (float)ao_data.samplerate;
 
         if ((err = snd_pcm_drop(alsa_handler)) < 0)
         {
@@ -757,6 +761,7 @@ static void reset(void)
     int err;
 
     prepause_frames = 0;
+    delay_before_pause = 0;
     if ((err = snd_pcm_drop(alsa_handler)) < 0)
     {
 	mp_tmsg(MSGT_AO,MSGL_ERR,"[AO_ALSA] pcm prepare error: %s\n", snd_strerror(err));
@@ -847,6 +852,9 @@ static float get_delay(void)
   if (alsa_handler) {
     snd_pcm_sframes_t delay;
 
+    if (snd_pcm_state(alsa_handler) == SND_PCM_STATE_PAUSED)
+      return delay_before_pause;
+
     if (snd_pcm_delay(alsa_handler, &delay) < 0)
       return 0;