wasapi: Initial WASAPI support, for Windows Vista and later.

This should remain binary compatible with Windows XP, as we dynamically
load anything we need and fall back to DirectSound/WinMM/XAudio2 if not
available.

include/SDL_config_windows.h

@@ -154,6 +154,7 @@ typedef unsigned int uintptr_t;
 #endif
 
 /* Enable various audio drivers */
+#define SDL_AUDIO_DRIVER_WASAPI 1
 #define SDL_AUDIO_DRIVER_DSOUND 1
 #define SDL_AUDIO_DRIVER_XAUDIO2    1
 #define SDL_AUDIO_DRIVER_WINMM  1