audio: Port WASAPI to WinRT, remove XAudio2 backend.

XAudio2 doesn't have capture support, so WASAPI was to replace it; the holdout
was WinRT, which still needed it as its primary audio target until the WASAPI
code code be made to work.

The support matrix now looks like:

WinXP: directsound by default, winmm as a fallback for buggy drivers.
Vista+: WASAPI (directsound and winmm as fallbacks for debugging).
WinRT: WASAPI

include/SDL_config_winrt.h

@@ -175,7 +175,7 @@ typedef unsigned int uintptr_t;
 #define HAVE__FSEEKI64 1
 
 /* Enable various audio drivers */
-#define SDL_AUDIO_DRIVER_XAUDIO2    1
+#define SDL_AUDIO_DRIVER_WASAPI 1
 #define SDL_AUDIO_DRIVER_DISK   1
 #define SDL_AUDIO_DRIVER_DUMMY  1