racket/gui win32: reset canvas backing size when scroll bars change

Avoids a gray box at the bottom of the GRacket REPL window on
startup, or in DrRacket when a horizontal scroll is automatically
removed due to, say, hiding a interactions window positioned to
the right of the definitions window (where the definitions content
is narrower than the whole window but wider than half).

Merge to v6.0
(cherry picked from commit 836f4cab20)

pkgs/gui-pkgs/gui-lib/mred/private/wx/win32/canvas.rkt

@@ -394,7 +394,8 @@
        (when vscroll?
          (atomically
           (set! v-scroll-visible? (and v? #t))
-          (ShowScrollBar canvas-hwnd SB_VERT v?))))
+          (ShowScrollBar canvas-hwnd SB_VERT v?)))
+       (reset-dc))
 
      (define/override (do-set-scrollbars h-step v-step
                                          h-len v-len