macos: set opengl view as opaque in opaque windows

We don't need the OpenGL view to be transparent if the window itself
is not transparent. This has the potential to speed up the compositing
of the GL view onto the NSWindow.

gdk/macos/GdkMacosGLView.c

@@ -82,7 +82,9 @@ G_GNUC_BEGIN_IGNORE_DEPRECATIONS
 
 -(BOOL)isOpaque
 {
-  return NO;
+  if ([self window])
+    return [[self window] isOpaque];
+  return YES;
 }
 
 -(BOOL)isFlipped