Allow playing iPod music in the background of SDL applications.

You can always change your audio session category afterwards if you want custom behavior.

src/audio/coreaudio/SDL_coreaudio.c

@@ -538,6 +538,14 @@ COREAUDIO_Init(SDL_AudioDriverImpl * impl)
     impl->DetectDevices = COREAUDIO_DetectDevices;
 #else
     impl->OnlyHasDefaultOutputDevice = 1;
+
+    /* Set category to ambient sound so that other music continues playing.
+       You can change this at runtime in your own code if you need different
+       behavior.  If this is common, we can add an SDL hint for this.
+    */
+    AudioSessionInitialize(NULL, NULL, NULL, nil);
+    UInt32 category = kAudioSessionCategory_AmbientSound;
+    AudioSessionSetProperty(kAudioSessionProperty_AudioCategory, sizeof(UInt32), &category);
 #endif
 
     impl->ProvidesOwnCallbackThread = 1;

src/audio/coreaudio/SDL_coreaudio.h

@@ -35,6 +35,8 @@
 #if MAC_OS_X_VERSION_MAX_ALLOWED <= 1050
 #include <AudioUnit/AUNTComponent.h>
 #endif
+#else
+#include <AudioToolbox/AudioToolbox.h>
 #endif
 
 #include <AudioUnit/AudioUnit.h>