Accumulating a big path with 'rlineto', a too big error is accumulated due to 
current point is represented in 'fixed' with 1/256 pixel precision in device 
space. It should be more precise. A precise result can be riched with 
representing current point in user space.

I don't like hacks here. I would prefer to fix it at once forever. Therefore 
the right solution is to add the current point to gs_imager_state and maintain 
it properly in :

- currentpoint;
- moveto, lineto, curveto, closepath;
- rmoveto, rlineto, rcurveto;
- arc, arcn, arct, arcto;
- gsave, grestore;
- save, restore;
- show, ashow, widthshow, awidthshow, xshow, xyshow, yshow, glyphshow, cshow, 
kshow;
- strokepath (?);
- charpath, uappend;

What do I miss ?