macOS: Keep default NSWindow background unless window is frameless

The logic was changed in ee82f8661 to only draw a window background
when the window was textured, and otherwise ask for a clear window
background. This has an unfortunate side-effect on macOS 10.15 that
the window's title bar will be partially transparent and reflect the
content under the window, with a blur effect.

It also inadvertently broke the use-case of setting the NSWindow
background color explicitly.

With this patch we're back to the behavior before ee82f8661, and
users who still want to have a non-borderless window with a clear
background can still do this by setting the background color to
the clear color manually.

Task-number: QTBUG-77637
Change-Id: I8a11bc46e6393b29a37f002ea123a987048106b9
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/plugins/platforms/cocoa/qnswindow.mm

@@ -255,8 +255,8 @@ static bool isMouseEvent(NSEvent *ev)
 
 - (NSColor *)backgroundColor
 {
-    return self.styleMask & NSWindowStyleMaskTexturedBackground ?
-                [super backgroundColor] : [NSColor clearColor];
+    return self.styleMask == NSWindowStyleMaskBorderless ?
+        [NSColor clearColor] : [super backgroundColor];
 }
 
 - (void)sendEvent:(NSEvent*)theEvent