Honor border width in a welded color picker for gtk3/x11

Change-Id: I1b59e85edb8e9df5be453dc0be1d7d1ffd100ceb
Reviewed-on: https://gerrit.libreoffice.org/56996
Tested-by: Jenkins
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>

vcl/unx/gtk3/gtk3gtkinst.cxx

@@ -2939,6 +2939,9 @@ private:
         }
         else
         {
+            //set border width
+            gtk_container_set_border_width(GTK_CONTAINER(m_pMenuHack), gtk_container_get_border_width(GTK_CONTAINER(m_pPopover)));
+
             //steal popover contents and smuggle into toplevel display window
             GtkWidget* pChild = gtk_bin_get_child(GTK_BIN(m_pPopover));
             g_object_ref(pChild);