add `get-device-scale' to dc<%>

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

@@ -151,6 +151,9 @@
     (define/override (get-size)
       (values (/ page-width page-scaling) (/ page-height page-scaling)))
 
+    (define/override (get-device-scale)
+      (values page-scaling page-scaling))
+
     (define current-page 0)
 
     (define/public (get-page-count) (length pages))

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

@@ -222,6 +222,9 @@
     (define/override (get-size)
       (values (/ page-width page-scaling) (/ page-height page-scaling)))
 
+    (define/override (get-device-scale)
+      (values page-scaling page-scaling))
+
     (define/override (end-doc)
       (send (new printout% 
                  [op-gtk (gtk_print_operation_new)]

collects/racket/draw/private/dc-intf.rkt

@@ -32,6 +32,7 @@
     get-char-height
     get-char-width
     get-clipping-region
+    get-device-scale
     get-font
     get-gl-context
     get-initial-matrix

collects/racket/draw/private/dc.rkt

@@ -129,8 +129,9 @@
     ;;  the color is for a background.
     install-color
 
-    ;; The public get-size method:
+    ;; The public get-size & get-device-scale methods:
     get-size
+    get-device-scale
 
     ;; set-auto-scroll : real real -> void
     ;;
@@ -197,6 +198,7 @@
     (define/public (collapse-bitmap-b&w?) #f)
 
     (define/public (get-size) (values 0.0 0.0))
+    (define/public (get-device-scale) (values 1.0 1.0))
 
     (define/public (set-auto-scroll dx dy) (void))
 
@@ -612,6 +614,10 @@
       (check-ok 'get-size)
       (super get-size))
 
+    (define/override (get-device-scale)
+      (check-ok 'get-device-scale)
+      (super get-device-scale))
+
     (def/public (suspend-flush) (void))
     (def/public (resume-flush) (void))
     (def/public (flush) (void))

collects/racket/draw/private/post-script-dc.rkt

@@ -142,6 +142,9 @@
             (values h w)
             (values w h))))
 
+    (define/override (get-device-scale)
+      (values scale-x scale-y))
+
     (define/override (end-cr)
       (cairo_surface_finish s)
       (cairo_destroy c)

collects/scribblings/draw/dc-intf.scrbl

@@ -540,6 +540,21 @@ Gets the current clipping region, returning @scheme[#f] if the drawing
 
 }
 
+
+@defmethod[(get-device-scale)
+           (values (and/c real? (not/c negative?))
+                   (and/c real? (not/c negative?)))]{
+
+Gets an ``external'' scaling factor for drawing coordinates to the
+target device. For most DCs, the result is @racket[1.0] and
+@racket[1.0].
+
+A @racket[post-script-dc%] or @racket[pdf-dc%] object returns scaling
+factors determined via @xmethod[ps-setup% get-scaling] at the time
+that the DC was created. A @racket[printer-dc%] may also have a
+user-configured scaling factor.}
+
+
 @defmethod[(get-font)
            (is-a?/c font%)]{