fix some uses of not-yet-defined fields

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

@@ -366,6 +366,10 @@
      (define is-gl? (and (not is-combo?) (memq 'gl style)))
      (define/public (can-gl?) is-gl?)
 
+     (define dc #f)
+     (define blits null)
+     (define reg-blits null)
+
      (super-new 
       [parent parent]
       [cocoa
@@ -421,7 +425,7 @@
        (tellv content-cocoa setDelegate: content-cocoa)
        (install-control-font content-cocoa #f))
 
-     (define dc (make-object dc% this (memq 'transparent canvas-style)))
+     (set! dc (make-object dc% this (memq 'transparent canvas-style)))
 
      (send dc start-backing-retained)
 
@@ -864,9 +868,6 @@
      (define/public (is-flipped?)
        (tell #:type _BOOL (get-cocoa-content) isFlipped))
 
-     (define blits null)
-     (define reg-blits null)
-
      (define/private (suspend-all-reg-blits)
        (let ([cocoa-win (get-cocoa-window)])
          (for ([r (in-list reg-blits)])

pkgs/gui-pkgs/gui-lib/mred/private/wx/cocoa/window.rkt

@@ -461,6 +461,10 @@
                 cocoa
                 [no-show? #f])
 
+    (define is-on? #f)
+    (define accept-drag? #f)
+    (define accept-parent-drag? #f)
+
     (super-new)
 
     (queue-autorelease-flush)
@@ -522,7 +526,6 @@
 
     (define/public (get-eventspace) eventspace)
 
-    (define is-on? #f)
     (define/public (show on?)
       (atomically
        (unless (eq? (and on? #t) is-on?)
@@ -674,9 +677,6 @@
     (define/public (move x y)
       (internal-move x y))
 
-    (define accept-drag? #f)
-    (define accept-parent-drag? #f)
-
     (define/public (on-drop-file f) (void))
     (define/public (do-on-drop-file f)
       (if accept-drag?

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

@@ -116,8 +116,6 @@
 ;; methods:
 (define (canvas-mixin %)
   (class %
-    (super-new)
-
     (inherit request-canvas-flush-delay
              cancel-canvas-flush-delay
              queue-canvas-refresh-event
@@ -132,6 +130,8 @@
     ;; of queued paint:
     (define paint-queued #f) ; #f or (box #t)
 
+    (super-new)
+
     (define/override (queue-paint)
       ;; can be called from any thread, including the event-pump thread
       (unless paint-queued