Function: plotbox
Class: highlevel
Section: graphic
C-Name: rectbox
Prototype: vLGG
Help: plotbox(w,x2,y2): if the cursor is at position (x1,y1), draw a box
 with diagonal (x1,y1) and (x2,y2) in rectwindow w (cursor does not move).
Doc: let $(x1,y1)$ be the current position of the virtual cursor. Draw in the
 rectwindow $w$ the outline of the rectangle which is such that the points
 $(x1,y1)$ and $(x2,y2)$ are opposite corners. Only the part of the rectangle
 which is in $w$ is drawn. The virtual cursor does \emph{not} move.