wasapi: Handle lost audio device endpoints.

This gracefully recovers when a device format is changed, and will switch
to the new default device if the current one is unplugged, etc.

This does not handle when a new default device is added; it only notices
if the current default goes away. That will be fixed by implementing the
stubbed-out MMNotificationClient_OnDefaultDeviceChanged() function.

src/audio/wasapi/SDL_wasapi.h

@@ -39,6 +39,7 @@ struct SDL_PrivateAudioData
     HANDLE task;
     SDL_bool coinitialized;
     int framesize;
+    SDL_bool is_default_device;
 };
 
 #endif /* SDL_wasapi_h_ */