[luatex] Override image resolution?

[Patrick Gundlach]

Hello Peter,

> I used \pdfimageresolution a few years ago. Should still be accessible
> via \pdfvariable.
> 
> \pdfimageresolution 300

there is a variable mentioned in the manual, but it has no effect:

tex.pdfimageresolution = 300

Thanks a lot anyway!
   Patrick