Add support for Spatial Audio, #3444

This commit will add setting the mpv ao property to avfoundation in the
mpvInit method of the MPVController class. This causes IINA to use the
AVFoundation framework for audio (which supports spatial audio) instead
of the Core Audio framework.

iina/MPVController.swift

@@ -393,6 +393,11 @@ not applying FFmpeg 9599 workaround
     chkErr(mpv_set_option_string(mpv, MPVOption.ProgramBehavior.resetOnNextFile,
             "\(MPVOption.PlaybackControl.abLoopA),\(MPVOption.PlaybackControl.abLoopB)"))
 
+    // As mpv support for audio using the AVFoundation framework is new we enable it before applying
+    // user's settings. This allows a user to roll back to the Core Audio framework should a problem
+    // be encountered with the new code.
+    chkErr(mpv_set_property_string(mpv, MPVOption.Audio.ao, "avfoundation"))
+
     // Set user defined conf dir.
     if Preference.bool(for: .enableAdvancedSettings),
        Preference.bool(for: .useUserDefinedConfDir),