Added XAudio2 backend.

Windows audio backend with lower latency.

Audio never glitches on my machine but the number of buffers may be set too aggressively. If you run into problems try turning up NUM_BUFFERS in the h file and leave some feedback.

From my tests games seem to prefer filling the buffer with smaller chunks. For this to work the callback that fills the buffer needs to either spin or run async so I went with async.

git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@6371 8ced0084-cf51-0410-be5f-012b33b47a6e

Source/Plugins/Plugin_DSP_HLE/Src/ConfigDlg.cpp

@@ -155,6 +155,7 @@ bool DSPConfigDialogHLE::SupportsVolumeChanges(std::string backend)
 	//       too much just to enable/disable a stupid slider...
 	return (backend == BACKEND_DIRECTSOUND ||
 			backend == BACKEND_OPENAL ||
+			backend == BACKEND_XAUDIO2 ||
 			backend == BACKEND_PULSEAUDIO);
 }