fix save-file and get-argb-pixels for screen bitmaps

original commit: 674d2e524858398f715b391d565153ced0443e45

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

@@ -26,9 +26,17 @@
     (super-make-object (make-alternate-bitmap-kind w h))
 
     (define s
-      (cairo_quartz_surface_create CAIRO_FORMAT_ARGB32
-                                   w
-                                   h))
+      (let ([s (cairo_quartz_surface_create CAIRO_FORMAT_ARGB32
+                                            w
+                                            h)])
+        ;; initialize bitmap to empty - needed?
+        #;
+        (let ([cr (cairo_create s)])
+          (cairo_set_operator cr CAIRO_OPERATOR_CLEAR)
+          (cairo_set_source_rgba cr 1.0 1.0 1.0 1.0)
+          (cairo_paint cr)
+          (cairo_destroy cr))
+        s))
 
     (define/override (ok?) #t)
     (define/override (is-color?) #t)

collects/scribblings/gui/blurbs.rkt

@@ -4,7 +4,8 @@
            scribble/manual
            scribble/scheme
            scribble/decode
-           (for-label scheme/gui/base)
+           (for-label scheme/gui/base
+                      scheme/base)
            (for-syntax scheme/base))
 
   (provide (except-out (all-defined-out) p define-inline))