use pen's alpha color stuff in 2htdp/image

collects/2htdp/tests/test-image.rkt

@@ -882,6 +882,11 @@
       =>
       (rectangle 100 10 "solid" "blue"))
 
+(test (overlay (rectangle 100 10 'solid (color 255 0 0 0))
+               (rectangle 100 10 'solid (color 0 255 0 255))
+               (rectangle 100 10 'solid (color 0 0 255 0)))
+      =>
+      (rectangle 100 10 'solid (color 0 255 0)))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;

collects/mrlib/image-core.rkt

@@ -1065,7 +1065,8 @@ the mask bitmap and the original bitmap are all together in a single bytes!
       (make-object color% 
         (color-red color)
         (color-green color)
-        (color-blue color))))
+        (color-blue color)
+        (/ (color-alpha color) 255))))
 
 
 (define (pen->pen-obj/cache pen)

collects/teachpack/2htdp/scribblings/image.scrbl

@@ -1222,9 +1222,6 @@ This section lists predicates for the basic structures provided by the image lib
   The constructor, @racket[make-color], also accepts only three arguments, in which case
   the three arguments are used for the @racket[red], @racket[green], and @racket[blue] fields, and the
   @racket[alpha] field defaults to @racket[255].
-  
-  Unfortunately, not all of the primitives that accept or produce @racket[color] structs use the @racket[alpha]
-  field; only @racket[image->color-list] and @racket[color-list->bitmap] do.  
 }
 
 @defproc[(y-place? [x any/c]) boolean?]{