macOS: Correctly scale CALayer contentsRect for child NSViews

When flushing parts of a QBackingStore onto child QWindows, and we're
in layer-backed mode, we need to set the contentsRect of the layer
so that the layer will only show the part of the top-level-owned
backingstore image that's relevant for the child window.

Since the contentsRect is in unit coordinate system, we need to apply
a transform, but this must be based on the top level view's size, not
the image size, as the latter will be twice as big on a retina screen,
giving the wrong unit rect.

Change-Id: I7d6f378ed46a98272efb13406a2878ec1efe734e
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoabackingstore.mm

@@ -127,10 +127,11 @@ void QCocoaBackingStore::flush(QWindow *window, const QRegion &region, const QPo
         // FIXME: Figure out if there's a way to do partial updates
         view.layer.contents = (__bridge id)static_cast<CGImageRef>(cgImage);
         if (view != topLevelView) {
+            const CGSize topLevelSize = topLevelView.bounds.size;
             view.layer.contentsRect = CGRectApplyAffineTransform(
                 [view convertRect:view.bounds toView:topLevelView],
                 // The contentsRect is in unit coordinate system
-                CGAffineTransformMakeScale(1.0 / m_image.width(), 1.0 / m_image.height()));
+                CGAffineTransformMakeScale(1.0 / topLevelSize.width, 1.0 / topLevelSize.height));
         }
     } else {
         // Normally a NSView is drawn via drawRect, as part of the display cycle in the