fix GC blit resizing

Conversion of a bitmap to the specified size was broken and made more
broken by recent changes.

gui-lib/mred/private/wx/common/canvas-mixin.rkt

@@ -203,8 +203,8 @@
            (= (send on get-width) w)
            (= (send on get-height) h))
       on
-      (let ([bm (make-object bitmap% w h #:backing-scale (send on get-backing-scale))])
-        (let ([dc (make-object bitmap-dc% on)])
+      (let ([bm (make-object bitmap% w h #f #f (send on get-backing-scale))])
+        (let ([dc (make-object bitmap-dc% bm)])
           (send dc draw-bitmap-section on 0 0 on-x on-y w h)
           (send dc set-bitmap #f)
           bm))))