gnuplot development

Previously posted on the bugs list; reposted here since it includes a
patch.
I noticed that with filledcurves the filled area diverges from the drawn
line when viewed in gv. The path drawing code has a workaround for rounding
error in the postscript interpreter, but this is missing in the polygon
drawing. This patch changes the polygon drawing code so that they both
output the exact same sequence of lineto/rlineto.

(That a line is drawn around the filled area as a separate (but now
identical) path is quite wasteful, and even more so when the line width is
0, but I haven't looked into whether this would be a hard thing to change.)