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.h.cmake

@@ -202,7 +202,6 @@
 #cmakedefine HAVE_DDRAW_H @HAVE_DDRAW_H@
 #cmakedefine HAVE_DSOUND_H @HAVE_DSOUND_H@
 #cmakedefine HAVE_DINPUT_H @HAVE_DINPUT_H@
-#cmakedefine HAVE_XAUDIO2_H @HAVE_XAUDIO2_H@
 #cmakedefine HAVE_XINPUT_H @HAVE_XINPUT_H@
 #cmakedefine HAVE_DXGI_H @HAVE_DXGI_H@
 #cmakedefine HAVE_XINPUT_GAMEPAD_EX @HAVE_XINPUT_GAMEPAD_EX@
@@ -259,7 +258,6 @@
 #cmakedefine SDL_AUDIO_DRIVER_SUNAUDIO @SDL_AUDIO_DRIVER_SUNAUDIO@
 #cmakedefine SDL_AUDIO_DRIVER_WASAPI @SDL_AUDIO_DRIVER_WASAPI@
 #cmakedefine SDL_AUDIO_DRIVER_WINMM @SDL_AUDIO_DRIVER_WINMM@
-#cmakedefine SDL_AUDIO_DRIVER_XAUDIO2 @SDL_AUDIO_DRIVER_XAUDIO2@
 
 /* Enable various input drivers */
 #cmakedefine SDL_INPUT_LINUXEV @SDL_INPUT_LINUXEV@

include/SDL_config.h.in

@@ -259,7 +259,6 @@
 #undef SDL_AUDIO_DRIVER_SUNAUDIO
 #undef SDL_AUDIO_DRIVER_WASAPI
 #undef SDL_AUDIO_DRIVER_WINMM
-#undef SDL_AUDIO_DRIVER_XAUDIO2
 
 /* Enable various input drivers */
 #undef SDL_INPUT_LINUXEV

include/SDL_config_windows.h

@@ -179,7 +179,6 @@ typedef unsigned int uintptr_t;
 /* Enable various audio drivers */
 #define SDL_AUDIO_DRIVER_WASAPI 1
 #define SDL_AUDIO_DRIVER_DSOUND 1
-#define SDL_AUDIO_DRIVER_XAUDIO2    0
 #define SDL_AUDIO_DRIVER_WINMM  1
 #define SDL_AUDIO_DRIVER_DISK   1
 #define SDL_AUDIO_DRIVER_DUMMY  1

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