fix transparent-canvas detection

collects/mred/private/wx/cocoa/canvas.rkt

@@ -360,7 +360,7 @@
        (tellv content-cocoa setDelegate: content-cocoa)
        (install-control-font content-cocoa #f))
 
-     (define dc (make-object dc% this))
+     (define dc (make-object dc% this (memq 'transparent canvas-style)))
 
      (send dc start-backing-retained)
 

collects/mred/private/wx/cocoa/dc.rkt

@@ -23,11 +23,12 @@
 
 (define dc%
   (class backing-dc%
-    (init [(cnvs canvas)])
+    (init [(cnvs canvas)]
+          transparent?)
     (define canvas cnvs)
 
     (inherit end-delay)
-    (super-new [transparent? (not (send canvas get-canvas-background))])
+    (super-new [transparent? transparent?])
 
     (define gl #f)
     (define/override (get-gl-context)

collects/mred/private/wx/gtk/canvas.rkt

@@ -284,7 +284,7 @@
 
      (set-size x y w h)
      
-     (define dc (new dc% [canvas this]))
+     (define dc (new dc% [canvas this] [transparent? (memq 'transparent style)]))
 
      (define for-gl? (memq 'gl style))
      (when for-gl?

collects/mred/private/wx/gtk/dc.rkt

@@ -92,11 +92,12 @@
 
 (define dc%
   (class backing-dc%
-    (init [(cnvs canvas)])
+    (init [(cnvs canvas)]
+          transparent?)
     (inherit end-delay)
     (define canvas cnvs)
 
-    (super-new [transparent? (not (send canvas get-canvas-background))])
+    (super-new [transparent? transparent?])
 
     (define gl #f)
     (define/override (get-gl-context)

collects/mred/private/wx/win32/canvas.rkt

@@ -204,7 +204,7 @@
      (define/override (wndproc-for-ctlproc w msg wParam lParam default)
        (default w msg wParam lParam))
      
-     (define dc (new dc% [canvas this]))
+     (define dc (new dc% [canvas this] [transparent? (memq 'transparent style)]))
      (send dc start-backing-retained)
 
      (define/public (get-dc) dc)

collects/mred/private/wx/win32/dc.rkt

@@ -63,11 +63,12 @@
 
 (define dc%
   (class backing-dc%
-    (init [(cnvs canvas)])
+    (init [(cnvs canvas)]
+          transparent?)
     (inherit end-delay)
     (define canvas cnvs)
 
-    (super-new [transparent? (not (send canvas get-canvas-background))])
+    (super-new [transparent? transparent?])
 
     (define gl #f)
     (define/override (get-gl-context)