[gtk/wip/chergert/quartz4u] macos: track opacity from toplevel window

[Christian Hergert <chergert src gnome org>]

commit 09b3949715abb1b3e483761397a87e40dd5ae989
Author: Christian Hergert <chergert redhat com>
Date:   Fri May 29 17:15:58 2020 -0700

    macos: track opacity from toplevel window

 gdk/macos/GdkMacosCairoView.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/gdk/macos/GdkMacosCairoView.c b/gdk/macos/GdkMacosCairoView.c
index 7158f570ea..8f049e71f5 100644
--- a/gdk/macos/GdkMacosCairoView.c
+++ b/gdk/macos/GdkMacosCairoView.c
@@ -40,7 +40,9 @@
 
 -(BOOL)isOpaque
 {
-  return NO;
+  if ([self window])
+    return [[self window] isOpaque];
+  return YES;
 }
 
 -(BOOL)isFlipped