SDL: Stop using double buffering mixer on macOS

This mixer type was added in
943b4c2036 because "anything which
produces sampled data with high latency (like the MT-32 emulator)
will sound terribly", but as far as I can see (or reproduce), this
mixer doesn't do anything that would solve that problem, except
that it effectively doubles the size of the audio buffer so there's
less chance of an underflow due to slower-than-realtime synthesis
by the softsynth. But you don't need the overhead of a separate
thread to do that, you just need to increase the buffer size.

backends/platform/sdl/macosx/macosx.cpp

@@ -64,7 +64,7 @@ void OSystem_MacOSX::init() {
 void OSystem_MacOSX::initBackend() {
 	// Create the mixer manager
 	if (_mixer == 0) {
-		_mixerManager = new DoubleBufferSDLMixerManager();
+		_mixerManager = new SdlMixerManager();
 
 		// Setup and start mixer
 		_mixerManager->init();