Function: plotrmove
Section: graphic
C-Name: plotrmove
Prototype: vLGG
Help: plotrmove(w,dx,dy): move cursor to position (dx,dy) relative to the
 present position in the rectwindow w.
Doc: move the virtual cursor of the rectwindow $w$ to position
 $(x1+dx,y1+dy)$, where $(x1,y1)$ is the initial position of the cursor
 (i.e.~to position $(dx,dy)$ relative to the initial cursor).
