macOS: Make layer-backed mode the default

QWindows are still backed by NSViews, but these views render to a
CoreAnimation layer instead of directly to the top level NSWindow.

This is the direction Apple is going (and is the only available
option on iOS/tvOS), so we want to move away from the existing
code path as soon as possible.

The default can be reversed by setting QT_MAC_WANTS_LAYER=0, or the
_q_mac_wantsLayer property on QWindow.

[ChangeLog][macOS] Layer-backed mode is now the default for QWindow.

Change-Id: Ibb9cc7541b179cad215d0daee14aeb1b54be614c
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/cocoa/qnsview_drawing.mm

@@ -147,12 +147,12 @@
     // on and off is not a supported use-case, so this code is effectively
     // returning a constant for the lifetime of our QSNSView, which means
     // we don't care about emitting KVO signals for @"wantsLayer".
-    bool layerRequested = qt_mac_resolveOption(false, m_platformWindow->window(),
+    bool wantsLayer = qt_mac_resolveOption(true, m_platformWindow->window(),
         "_q_mac_wantsLayer", "QT_MAC_WANTS_LAYER");
 
     bool layerForSurfaceType = [self shouldUseMetalLayer:m_platformWindow->window()->surfaceType()];
 
-    return layerRequested || layerForSurfaceType;
+    return wantsLayer || layerForSurfaceType;
 }
 
 - (CALayer *)makeBackingLayer