libroot: Fix ICU locale implementation of wcsxfrm.

* The "size" parameter is the size of "out" not "in", and the
   return size_t parameter is supposed to always have the total amount
   of wchar_ts needed, not how many are actually used.
 * In the case where "outSize == 0", we set "requiredSize" and then
   return.

Fixes crashes seen in glib2 Unicode collation routines, which
are used in GTK file dialogs.

Thanks to PulkoMandy for glancing at this.

headers/private/libroot/locale/ICUCollateData.h

@@ -33,7 +33,7 @@ public:
 			status_t			Wcscoll(const wchar_t* a, const wchar_t* b,
 									int& out);
 			status_t			Wcsxfrm(wchar_t* out, const wchar_t* in,
-									size_t size, size_t& outSize);
+									size_t outSize, size_t& requiredSize);
 
 private:
 			status_t			_ToUnicodeString(const char* in,

headers/private/libroot/locale/ICULocaleBackend.h

@@ -64,7 +64,7 @@ public:
 	virtual	status_t			Wcscoll(const wchar_t* a, const wchar_t* b,
 									int& out);
 	virtual status_t			Wcsxfrm(wchar_t* out, const wchar_t* in,
-									size_t size, size_t& outSize);
+									size_t outSize, size_t& requiredSize);
 
 	virtual status_t			TZSet(const char* timeZoneID, const char* tz);
 	virtual	status_t			Localtime(const time_t* inTime,

headers/private/libroot/locale/LocaleBackend.h

@@ -147,7 +147,7 @@ public:
 	virtual	status_t			Wcscoll(const wchar_t* a, const wchar_t* b,
 									int& out) = 0;
 	virtual status_t			Wcsxfrm(wchar_t* out, const wchar_t* in,
-									size_t size, size_t& outSize) = 0;
+									size_t outSize, size_t& requiredSize) = 0;
 
 	virtual status_t			TZSet(const char* timeZoneID,
 									const char* tz) = 0;