Function: plotrline
Section: graphic
C-Name: plotrline
Prototype: vLGG
Help: plotrline(w,dx,dy): if the cursor is at (x1,y1), draw a line from
 (x1,y1) to (x1+dx,y1+dy) (and move the cursor) in the rectwindow w.
Doc: draw in the rectwindow $w$ the part of the segment
 $(x1,y1)-(x1+dx,y1+dy)$ which is inside $w$, where $(x1,y1)$ is the current
 position of the virtual cursor, and move the virtual cursor to
 $(x1+dx,y1+dy)$ (even if it is outside the window).
