windows: directsound should also map audio device GUIDs to proper names.

Moved this code from winmm into core so both can use it.

DirectSound (at least on Win10) also returns truncated device names, even
though it's handed in as a string pointer and not a static-sized buffer.  :/

src/core/windows/SDL_windows.h

@@ -59,6 +59,9 @@ extern void WIN_CoUninitialize(void);
 /* Returns SDL_TRUE if we're running on Windows Vista and newer */
 extern BOOL WIN_IsWindowsVistaOrGreater();
 
+/* You need to SDL_free() the result of this call. */
+extern char *WIN_LookupAudioDeviceName(const WCHAR *name, const GUID *guid);
+
 #endif /* _INCLUDED_WINDOWS_H */
 
 /* vi: set ts=4 sw=4 expandtab: */