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

docs/README-winrt.md

@@ -70,7 +70,10 @@ Here is a rough list of what works, and what doesn't:
     SDL_GetPerformanceFrequency(), etc.)
   * file I/O via SDL_RWops
   * mouse input  (unsupported on Windows Phone)
-  * audio, via a modified version of SDL's XAudio2 backend
+  * audio, via SDL's WASAPI backend (if you want to record, your app must 
+    have "Microphone" capabilities enabled in its manifest, and the user must 
+    not have blocked access. Otherwise, capture devices will fail to work,
+    presenting as a device disconnect shortly after opening it.)
   * .DLL file loading.  Libraries *MUST* be packaged inside applications.  Loading
     anything outside of the app is not supported.
   * system path retrieval via SDL's filesystem APIs