Function: plotrbox
Class: highlevel
Section: graphic
C-Name: rectrbox
Prototype: vLGG
Help: plotrbox(w,dx,dy): if the cursor is at (x1,y1), draw a box with
 diagonal (x1,y1)-(x1+dx,y1+dy) in rectwindow w (cursor does not move).
Doc: draw in the rectwindow $w$ the outline of the rectangle which is such
 that the points $(x1,y1)$ and $(x1+dx,y1+dy)$ are opposite corners, where
 $(x1,y1)$ is the current position of the cursor. Only the part of the
 rectangle which is in $w$ is drawn. The virtual cursor does \emph{not} move.