W32: Don't crash when opacity is set for non-toplevel

X11 backend doesn't, and for good reason - main code body does not check
that the window it sets opacity for is, in fact, toplevel.
Just silently fail to do anything for non-toplevel windows.

https://bugzilla.gnome.org/show_bug.cgi?id=733769

gdk/win32/gdkwindow-win32.c

@@ -3175,9 +3175,8 @@ gdk_win32_window_set_opacity (GdkWindow *window,
   PFN_SetLayeredWindowAttributes setLayeredWindowAttributes = NULL;
 
   g_return_if_fail (GDK_IS_WINDOW (window));
-  g_return_if_fail (WINDOW_IS_TOPLEVEL (window));
 
-  if (GDK_WINDOW_DESTROYED (window))
+  if (!WINDOW_IS_TOPLEVEL (window) || GDK_WINDOW_DESTROYED (window))
     return;
 
   if (opacity < 0)