ao_coreaudio: use AudioUnitReset as ao_driver.reset to prevent long r…

…estart

[motivation]
Seeking on MacOS appears to be lagged when users connect
to wireless audio output (airpods for example).

This commit attempts to fix mpv-player#10270

[observation]
1. When using other media player (VLC to be exact) simultaneously,
the lagging on seek disappear. We could guess that the AudioDevice
is on some sort of "warm-up" state.

See mpv-player#9243 for detailed description.

2. `AudioOutputUnitStart` takes significant longer time after each seek
or pause/play when using wireless output devices compares to wired devices.

[rationale]
After investigate codes in ao_coreaudio.c, it appears that the the `stop`
function was used as `ao_driver.reset` function. Therefore every seek
and pause would call `AudioOutputUnitStop`.

It turns out that `ao_driver.reset` function is used in `ao_reset`.
And `ao_reset` function is used to clean up the state of current `ao`
so I think `AudioUnitReset` is more proper than `AudioOutputUnitStop`
under this semantics.

Since ao_coreaudio use pull base mechanism, audio playback behaviors
upon pause/seek could be handled by callback function
(streaming silence when paused) so there is no need to stop AudioUnit when resetting.
Therefore using `AudioUnitReset` as `ao_driver.reset` looks proper.

Additionally, after using proper reset, the AudioUnit that represents
hardware I/O devices doesn't need to be restart everytime seek/pause actions happen.
Restarting wireless devices simply takes longer in MacOS which is
the root cause of lagging observed by users when they seek or pause/play media.

[method]
Use `AudioUnitReset` for ao_driver.reset.

audio/out/ao_coreaudio.c

@@ -314,11 +314,11 @@ static bool init_audiounit(struct ao *ao, AudioStreamBasicDescription asbd)
     return false;
 }
 
-static void stop(struct ao *ao)
+static void reset(struct ao *ao)
 {
     struct priv *p = ao->priv;
-    OSStatus err = AudioOutputUnitStop(p->audio_unit);
-    CHECK_CA_WARN("can't stop audio unit");
+    OSStatus err = AudioUnitReset(p->audio_unit, kAudioUnitScope_Global, 0);
+    CHECK_CA_WARN("can't reset audio unit");
 }
 
 static void start(struct ao *ao)
@@ -415,7 +415,7 @@ const struct ao_driver audio_out_coreaudio = {
     .uninit         = uninit,
     .init           = init,
     .control        = control,
-    .reset          = stop,
+    .reset          = reset,
     .start          = start,
     .hotplug_init   = hotplug_init,
     .hotplug_uninit = hotplug_uninit,