ASIO backend, part 2

half-working:
- crashes on exit
- doesn't work on launch (must go to settings > audio and click OK)

src/engine/engine.h

@@ -76,6 +76,7 @@ enum DivAudioEngines {
   DIV_AUDIO_SDL=1,
   DIV_AUDIO_PORTAUDIO=2,
   DIV_AUDIO_PIPE=3,
+  DIV_AUDIO_ASIO=4,
 
   DIV_AUDIO_NULL=126,
   DIV_AUDIO_DUMMY=127