I've tried implementing this before but importing user32 from ntdll isn't a reliable
thing (so I recently rewrote it using the ntdll equivalents of LoadLibrary() and GetProcAddress())
Note: This is a weird way to implement this function (Windows implements this
in ntdll instead) but that would probably require a user32/ntdll rewrite (so this
is the next best tning to get the application working)
--
v4: ntdll: Implement NtdllDefWindowProc_A().
https://gitlab.winehq.org/wine/wine/-/merge_requests/5236