Comment 4 for bug 391923

fix tested and committed to svn rev 21716, thanks Preben!

I did not quite understand the implementation of the x,y offset so I took the liberty of rewriting the x,y offset as a displaced origin instead, in units of the screen display pixels. However, I can see that there is no easy way to deal with the upside down coordinate system, so feel free to rewrite my rewrite, if desired.