ao_audiounit: use non-exclusive audio session

Using this option means the OS won't pause other
sources of audio when playback starts in MPV.

But if --audio-exclusive is used, still use an exclusive session.

This should apply to iOS, iPadOS, and tvOS.

DOCS/man/options.rst

@@ -1900,8 +1900,8 @@ Audio
     Enable exclusive output mode. In this mode, the system is usually locked
     out, and only mpv will be able to output audio.
 
-    This only works for some audio outputs, such as ``wasapi``, ``coreaudio``
-    and ``pipewire``. Other audio outputs silently ignore this option.
+    This only works for some audio outputs, such as ``wasapi``, ``coreaudio``,
+    ``pipewire`` and ``audiounit``. Other audio outputs silently ignore this option.
     They either have no concept of exclusive mode, or the mpv side of the
     implementation is missing.
 

audio/out/ao_audiounit.m

@@ -115,7 +115,12 @@ static bool init_audiounit(struct ao *ao)
 
     MP_VERBOSE(ao, "max channels: %ld, requested: %d\n", maxChannels, (int)ao->channels.num);
 
-    [instance setCategory:AVAudioSessionCategoryPlayback error:nil];
+    AVAudioSessionCategoryOptions options = 0;
+    if (!(ao->init_flags & AO_INIT_EXCLUSIVE)) {
+        options |= AVAudioSessionCategoryOptionMixWithOthers;
+    }
+
+    [instance setCategory:AVAudioSessionCategoryPlayback withOptions:options error:nil];
     [instance setMode:AVAudioSessionModeMoviePlayback error:nil];
     [instance setActive:YES error:nil];
     [instance setPreferredOutputNumberOfChannels:prefChannels error:nil];