Fix CSD check for Win32

Win32 does not have alpha channel currently ; fix the check
for this, so trying to enable CSDs on this platform will
not "succeed" and crash the app anymore.

Partially fixes gtk3-widget-factory.

gtk/gtkwindow.c

@@ -61,6 +61,10 @@
 #include "x11/gdkx.h"
 #endif
 
+#ifdef GDK_WINDOWING_WIN32
+#include "win32/gdkwin32.h"
+#endif
+
 #ifdef GDK_WINDOWING_WAYLAND
 #include "wayland/gdkwayland.h"
 #endif
@@ -3507,6 +3511,21 @@ gdk_window_supports_csd (GtkWindow *window)
     }
 #endif
 
+#ifdef GDK_WINDOWING_WIN32
+  if (GDK_IS_WIN32_DISPLAY (gtk_widget_get_display (widget)))
+    {
+      GdkScreen *screen;
+      GdkVisual *visual;
+
+      screen = gtk_widget_get_screen (widget);
+
+      /* We need a visual with alpha */
+      visual = gdk_screen_get_rgba_visual (screen);
+      if (!visual)
+        return FALSE;
+    }
+#endif
+
   return TRUE;
 }