#648 Updated the GetWindowLongPtr and SetWindowLongPtrW index

[Xileer Torias]

Updated the GetWindowLongPtr and SetWindowLongPtrW nIndex reference as per https://msdn.microsoft.com/en-us/library/windows/desktop/ms633585%28v=vs.85%29.aspx and https://msdn.microsoft.com/en-us/library/windows/desktop/ms644898(v=vs.85).aspx

This alters the function to be compatible with both x86, and x64 as to simplify future x64 builds.