Cocoa: Update layer contentsScale on screen change

Failure to do so will result in displaying the layer
contents on a quarter of the window when moving the
window from a 2x screen to a 1x screen.

Task-number: QTBUG-64494
Change-Id: I57ce98025e841549f62d132d4985e727898e6207
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qnsview.mm

@@ -353,6 +353,12 @@ Q_LOGGING_CATEGORY(lcQpaTablet, "qt.qpa.input.tablet")
     m_platformWindow->handleExposeEvent(QRectF::fromCGRect(self.bounds).toRect());
 }
 
+- (void)viewDidChangeBackingProperties
+{
+    if (self.layer)
+        self.layer.contentsScale = self.window.backingScaleFactor;
+}
+
 - (BOOL)isFlipped
 {
     return YES;