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

original commit: 836f4cab20c9ac4e8084a0c9f97e10e82533809f

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