Avoid "Gtk-WARNING **: State doesn't match"

See https://github.com/wxWidgets/wxWidgets/pull/232
(backport of 69a13d973c)

src/gtk/window.cpp

@@ -304,6 +304,7 @@ draw_border(GtkWidget* widget, GdkEventExpose* gdk_event, wxWindow* win)
 #ifdef __WXGTK3__
         GtkStyleContext* sc = gtk_widget_get_style_context(win->m_wxwindow);
         GdkRGBA c;
+        gtk_style_context_set_state(sc, GTK_STATE_FLAG_NORMAL);
         gtk_style_context_get_border_color(sc, GTK_STATE_FLAG_NORMAL, &c);
         gdk_cairo_set_source_rgba(cr, &c);
         cairo_set_line_width(cr, 1);
@@ -4443,6 +4444,7 @@ void wxWindowGTK::GTKApplyStyle(GtkWidget* widget, GtkRcStyle* WXUNUSED_IN_GTK3(
     cairo_pattern_t* pattern = NULL;
     if (m_backgroundColour.IsOk())
     {
+        gtk_style_context_set_state(context, GTK_STATE_FLAG_NORMAL);
         gtk_style_context_get(context,
             GTK_STATE_FLAG_NORMAL, "background-image", &pattern, NULL);
     }