fix get-extent signature

Author: rfindler

htdp-lib/graphics/private/value-turtles.rkt

@@ -224,7 +224,7 @@
         (send dc set-pen old-pen)
         (send dc set-brush old-brush)
         (send dc set-clipping-region old-clip)))
-    (define/override (get-extent dc x y w h descent space lspace rspace)
+    (define/override (get-extent dc x y [w #f] [h #f] [descent #f] [space #f] [lspace #f] [rspace #f])
       (set-box/f w (+ width 2))
       (set-box/f h (+ height 2))
       (set-box/f descent 0)

htdp-lib/stepper/private/mred-extensions.rkt

@@ -82,7 +82,7 @@
         (send s set-style (get-style))
         s))
 
-    (define (get-extent dc x y w-box h-box descent-box space-box lspace-box rspace-box)
+    (define (get-extent dc x y [w-box #f] [h-box #f] [descent-box #f] [space-box #f] [lspace-box #f] [rspace-box #f])
       (for-each (lambda (box) (unless (not box) (set-box! box 0)))
                 (list descent-box space-box lspace-box rspace-box))
       (unless (not w-box)