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

CMakeLists.txt

@@ -1180,11 +1180,10 @@ elseif(WINDOWS)
     check_include_file(ddraw.h HAVE_DDRAW_H)
     check_include_file(dsound.h HAVE_DSOUND_H)
     check_include_file(dinput.h HAVE_DINPUT_H)
-    check_include_file(xaudio2.h HAVE_XAUDIO2_H)
     check_include_file(mmdeviceapi.h HAVE_MMDEVICEAPI_H)
     check_include_file(audioclient.h HAVE_AUDIOCLIENT_H)
     check_include_file(dxgi.h HAVE_DXGI_H)
-    if(HAVE_D3D_H OR HAVE_D3D11_H OR HAVE_DDRAW_H OR HAVE_DSOUND_H OR HAVE_DINPUT_H OR HAVE_XAUDIO2_H)
+    if(HAVE_D3D_H OR HAVE_D3D11_H OR HAVE_DDRAW_H OR HAVE_DSOUND_H OR HAVE_DINPUT_H)
       set(HAVE_DIRECTX TRUE)
       if(NOT CMAKE_COMPILER_IS_MINGW AND NOT USE_WINSDK_DIRECTX)
       # TODO: change $ENV{DXSDL_DIR} to get the path from the include checks
@@ -1207,12 +1206,6 @@ elseif(WINDOWS)
       set(SOURCE_FILES ${SOURCE_FILES} ${DSOUND_AUDIO_SOURCES})
     endif()
 
-    if(HAVE_XAUDIO2_H)
-      set(SDL_AUDIO_DRIVER_XAUDIO2 1)
-      file(GLOB XAUDIO2_AUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/xaudio2/*.c)
-      set(SOURCE_FILES ${SOURCE_FILES} ${XAUDIO2_AUDIO_SOURCES})
-    endif()
-
     if(HAVE_AUDIOCLIENT_H AND HAVE_MMDEVICEAPI_H)
       set(SDL_AUDIO_DRIVER_WASAPI 1)
       file(GLOB WASAPI_AUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/wasapi/*.c)