[luatex] More on loading non-ascii filenames on windows

[Patrick Gundlach]

Hello David,

> Ulrike's example also works in cygwin texlive 2018 on windows
> 
> It also works if you use the short name (as returned by dir /x cöw.pdf )
> 
> a = img.write({ filename = "CW9463\string~1.PDF"})

 
that doesn't work. I have tried that (with the WinAPI call GetShortPathName), but that fails because Windows does not always return a short name, sometimes it just returns the name itself (depending on the file system and if a registry setting). 

Patrick