racket/draw: add make-dc' convenience method to bitmap%'

collects/racket/draw/private/bitmap.rkt

@@ -22,10 +22,15 @@
          make-platform-bitmap
          read-bitmap
          make-monochrome-bitmap
+         -bitmap-dc%
          (protect-out make-alternate-bitmap-kind
                       build-cairo-surface
                       quartz-bitmap%
-                      win32-no-hwnd-bitmap%))
+                      win32-no-hwnd-bitmap%
+                      install-bitmap-dc-class!))
+
+(define -bitmap-dc% #f)
+(define (install-bitmap-dc-class! v) (set! -bitmap-dc% v))
 
 ;; FIXME: there must be some way to abstract over all many of the
 ;; ARGB/RGBA/BGRA iterations.
@@ -286,6 +291,8 @@
     (define/public (get-bitmap-gl-context)
       #f)
 
+    (define/public (make-dc) (make-object -bitmap-dc% this))
+
     (define/public (load-file in
                               [kind 'unknown]
                               [bg #f]

collects/racket/draw/private/dc.rkt

@@ -26,7 +26,6 @@
 (provide dc-mixin
          dc-backend<%>
          default-dc-backend%
-         install-bitmap-dc-class!
          do-set-pen!
          do-set-brush!)
 
@@ -45,9 +44,6 @@
           black)
       (color->immutable-color c)))
 
-(define -bitmap-dc% #f)
-(define (install-bitmap-dc-class! v) (set! -bitmap-dc% v))
-
 ;; dc-backend : interface
 ;;
 ;; This is the interface that the backend specific code must implement

collects/scribblings/draw/bitmap-class.scrbl

@@ -241,6 +241,12 @@ For PNG loading, if @racket[bg-color] is not @racket[#f], then it is
 
 }
 
+@defmethod[(make-dc)
+           boolean?]{
+
+Return @racket[(make-object bitmap-dc% this)].}
+
+
 @defmethod[(ok?)
            boolean?]{
 

collects/scribblings/draw/guide.scrbl

@@ -11,13 +11,13 @@
                                      (let ([w (send bm0 get-width)]
                                            [h (send bm0 get-height)])
                                        (let ([bm (make-bitmap w h)])
-                                         (let ([dc (make-object bitmap-dc% bm)])
+                                         (let ([dc (send bm make-dc)])
                                            (send dc draw-bitmap bm0 0 0)
                                            (send dc set-bitmap #f))
                                           bm)))]
 @interaction-eval[#:eval draw-eval (define (line-bitmap mode)
                                      (let* ([bm (make-bitmap 30 4)]
-                                            [dc (make-object bitmap-dc% bm)])
+                                            [dc (send bm make-dc)])
                                        (send dc set-smoothing mode)
                                        (send dc draw-line 0 2 30 2)
                                        (send dc set-bitmap #f)