core-graphics: set premultiplied-alpha? in make-bitmap-image. Fixes #73

basis/core-graphics/core-graphics.factor

@@ -155,4 +155,5 @@ PRIVATE>
 : make-bitmap-image ( dim quot -- image )
     '[ <CGBitmapContext> &CGContextRelease @ ] make-memory-bitmap
     ARGB >>component-order
-    ubyte-components >>component-type ; inline
+    ubyte-components >>component-type
+    t >>premultiplied-alpha? ; inline

basis/images/cocoa/cocoa.factor

@@ -57,8 +57,7 @@ PRIVATE>
     image CGImageGetHeight :> h
     { w h } [
         0 0 w h <CGRect> image CGContextDrawImage
-    ] make-bitmap-image
-    t >>premultiplied-alpha? ;
+    ] make-bitmap-image ;
 
 M: ns-image stream>image
     drop stream-contents <CGImage> CGImage>image ;