racket/draw: faster draw-bitmap when alpha < 1.0

In common cases, use Cairo's alpha support instead of cloning the
bitmap to apply an alpha.

original commit: c3eae40f4c3dcda84096a544885751205783c1cd

pkgs/gui-pkgs/gui-test/tests/gracket/draw.rkt

@@ -12,7 +12,7 @@
 
 (define sys-path 
   (lambda (f)
-    (build-path (collection-path "icons") f)))
+    (collection-file-path f "icons")))
 
 (define local-path 
   (let ([d (current-load-relative-directory)])