macOS: set dpr on pixmap returned by grabWindow()

This matches the behavior of QScreen::grabWindow(),
and gives the caller direct access to the scale factor.

Change-Id: Ia3ed165a62eaa0f386f8b508ea6b1128ba6be604
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoascreen.mm

@@ -466,6 +466,7 @@ QPixmap QCocoaScreen::grabWindow(WId window, int x, int y, int width, int height
 
     const qreal dpr = devicePixelRatio();
     QPixmap windowPixmap(windowSize * dpr);
+    windowPixmap.setDevicePixelRatio(dpr);
     windowPixmap.fill(Qt::transparent);
 
     for (uint i = 0; i < displayCount; ++i) {