correct backing scale on Retina canvas & derived contexts

One consequence is that drawing with a pen of width 0 into
a canvas% object on a Retina display draws a line that is
half as wide as a pen of width 1.

pkgs/draw-pkgs/draw-doc/scribblings/draw/pen-class.scrbl

@@ -55,15 +55,15 @@ To avoid creating multiple pens with the same characteristics, use the
  global @racket[pen-list%] object @indexed-racket[the-pen-list], or
  provide a color, width, and style to @xmethod[dc<%> set-pen].
 
-When drawing in @racket['smoothed] or @racket['aligned] mode, a pen's
+When drawing in @racket['unsmoothed] or @racket['aligned] mode, a pen's
  size is truncated after scaling to an integral size. A pen of size
  @racket[0] (after truncation, if applicable) uses a non-zero,
  scale-insensitive line size for the destination drawing context:
  @racket[1/4] unit (after scaling) for @racket[post-script-dc%] or
  @racket[pdf-dc%] contexts in @racket['smoothed] mode, or @racket[1]
  unit (after scaling) for any other context.  For example, in unscaled 
- canvas and bitmap contexts, a zero-width pen behaves the same as a
- pen of size @racket[1].
+ canvas and bitmap contexts with a @tech{backing scale} of 1.0,
+ a zero-width pen behaves the same as a pen of size @racket[1].
 
 See also @racket[make-pen].
 

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

@@ -160,12 +160,16 @@
 
     (define is-trans? trans?)
 
-    (super-make-object w h trans? 1
-                       (let ([cg (CGLayerGetContext layer)])
-                         (unless flipped?
-                           (CGContextTranslateCTM cg 0 h)
-                           (CGContextScaleCTM cg 1 -1))
-                         cg))
+    (let ([bs (inexact->exact 
+               (display-bitmap-resolution 0 (lambda () 1)))])
+      (super-make-object w h trans? bs
+                         (let ([cg (CGLayerGetContext layer)])
+                           (unless flipped?
+                             (CGContextTranslateCTM cg 0 h)
+                             (CGContextScaleCTM cg 1 -1))
+                           (unless (= bs 1)
+                             (CGContextScaleCTM cg (/ 1 bs) (/ 1 bs)))
+                           cg)))
 
     (define/override (draw-bitmap-to cr sx sy dx dy w h alpha clipping-region)
       ;; Called when the destination rectangle is inside the clipping region