screen dc corrections and clarifications

original commit: 5ebfa781bdc4a6ee9bf41f01cef1c76dde36e78c

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

@@ -329,7 +329,7 @@
     (define/public (get-dc) dc)
 
     (define/public (make-compatible-bitmap w h)
-      (send dc make-backing-bitmap w h #t))
+      (send dc make-backing-bitmap w h))
 
     (define/override (get-client-gtk) client-gtk)
     (define/override (handles-events? gtk) (not (ptr-equal? gtk combo-button-gtk)))

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

@@ -38,6 +38,7 @@
 
     (define/override (ok?) #t)
     (define/override (is-color?) #t)
+    (define/override (has-alpha-channel?) #f)
     
     (define/override (get-cairo-surface) s)
 
@@ -80,15 +81,13 @@
 
     (super-new)
 
-    (define/override (make-backing-bitmap w h [any-bg? #f])
+    (define/override (make-backing-bitmap w h)
       (cond
        [(and (eq? 'unix (system-type))
-	     (or any-bg?
+             (send canvas get-canvas-background))
-		 (send canvas get-canvas-background)))
 	(make-object x11-bitmap% w h (widget-window (send canvas get-client-gtk)))]
        [(and (eq? 'windows (system-type))
-	     (or any-bg?
+             (send canvas get-canvas-background))
-		 (send canvas get-canvas-background)))
 	(make-object win32-bitmap% w h (widget-window (send canvas get-client-gtk)))]
        [else
 	(super make-backing-bitmap w h)]))

collects/scribblings/gui/miscwin-funcs.scrbl

@@ -294,8 +294,7 @@ environment of the result namespace.}
          (is-a/c? bitmap%)]{
 
 Creates a bitmap that draws in a way that is the same as drawing to a
-canvas in its default configuration. The bitmap is always in color
+canvas in its default configuration.
-with an alpha channel.
 
 A normal @racket[bitmap%] draws in a more platform-independent way and
 may use fewer constrained resources, particularly under Windows.}