coreaudio: Use the standard SDL audio thread instead of spinning a new one.

Fixes corner cases, like the audio callback not firing if the device is
disconnected, etc.

src/audio/coreaudio/SDL_coreaudio.h

@@ -45,16 +45,14 @@
 
 struct SDL_PrivateAudioData
 {
-    SDL_Thread *thread;
     AudioQueueRef audioQueue;
+    int numAudioBuffers;
     AudioQueueBufferRef *audioBuffer;
     void *buffer;
-    UInt32 bufferOffset;
     UInt32 bufferSize;
     AudioStreamBasicDescription strdesc;
-    SDL_sem *ready_semaphore;
-    char *thread_error;
-    SDL_atomic_t shutdown;
+    SDL_bool refill;
+    SDL_AudioStream *capturestream;
 #if MACOSX_COREAUDIO
     AudioDeviceID deviceID;
 #else