coreaudio: dynamically allocate AudioQueueBuffers.

We need more than two buffers to flip between if they are small, or CoreAudio
won't make any sound; apparently it needs X milliseconds of audio queued when
it needs to play more or it drops any queued buffers. We are currently
guessing 50 milliseconds as a minimum, but there's probably a more proper
way to get the minimum time period from the system.

Fixes Bugzilla #3656.

src/audio/coreaudio/SDL_coreaudio.h

@@ -47,7 +47,8 @@ struct SDL_PrivateAudioData
 {
     SDL_Thread *thread;
     AudioQueueRef audioQueue;
-    AudioQueueBufferRef audioBuffer[2];
+    int numAudioBuffers;
+    AudioQueueBufferRef *audioBuffer;
     void *buffer;
     UInt32 bufferOffset;
     UInt32 bufferSize;