This was added in 3f268cc4f2 because it wasn't clear whether
WM_NCCREATE could be used to send a context pointer from CreateWindowEx
to the window procedure. WM_NCCREATE isn't always the first message sent
to a window, but this seems to be because of an undocumented but
well-known bug where, for top-level overlapped windows, a spurious
WM_GETMINMAXINFO arrives first instead. The WM_GETMINMAXINFO can be
safely ignored, so it is still okay to pass the context struct in
WM_NCCREATE.
James Ross-Gowan
fe09d76142