macOS: Do layer updates via the CALayerDelegate protocol

The updateLayer function is only called when the layer is a
_NSViewBackingLayer instance, which is what the default
implementation of [NSView makeBackingLayer] returns.

Once we move to optionally returning a CAMetalLayer, we need
to use the more generic displayLayer: API, so we do that now
as a first step.

This matches the way we draw and send expose events on iOS.

Change-Id: I49721ff005ca9dfddebff645705f96b5ab46abb4
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/cocoa/qnsview.mm

@@ -74,7 +74,7 @@
 - (BOOL)isTransparentForUserInput;
 @end
 
-@interface QT_MANGLE_NAMESPACE(QNSView) (Drawing)
+@interface QT_MANGLE_NAMESPACE(QNSView) (Drawing) <CALayerDelegate>
 @end
 
 @interface QT_MANGLE_NAMESPACE(QNSViewMouseMoveHelper) : NSObject

src/plugins/platforms/cocoa/qnsview_drawing.mm

@@ -137,17 +137,14 @@
     }
 }
 
-- (BOOL)wantsUpdateLayer
+- (void)displayLayer:(CALayer *)layer
 {
-    return YES;
-}
+    Q_ASSERT(layer == self.layer);
 
-- (void)updateLayer
-{
     if (!m_platformWindow)
         return;
 
-    qCDebug(lcQpaDrawing) << "[QNSView updateLayer]" << m_platformWindow->window();
+    qCDebug(lcQpaDrawing) << "[QNSView displayLayer]" << m_platformWindow->window();
 
     // FIXME: Find out if there's a way to resolve the dirty rect like in drawRect:
     [self updateRegion:QRectF::fromCGRect(self.bounds).toRect()];