themingengine: don't hardcode white to draw insensitive labels shadow

Use the background color instead.
I think the optimal solution would be delegating the shadow rendering to
the theme completely, and removing this in GtkThemingEngine, but for
now, this simple fix makes the dark variant more usable.

https://bugzilla.gnome.org/show_bug.cgi?id=645405

gtk/gtkthemingengine.c

@@ -2402,8 +2402,14 @@ gtk_theming_engine_render_layout (GtkThemingEngine *engine,
 
   if (flags & GTK_STATE_FLAG_INSENSITIVE)
     {
+      GdkRGBA bg;
+
+      gtk_theming_engine_get_background_color (engine, flags, &bg);
+
       cairo_save (cr);
-      cairo_set_source_rgb (cr, 1, 1, 1);
+
+      gdk_cairo_set_source_rgba (cr, &bg);
+
       cairo_move_to (cr, x + 1, y + 1);
       _gtk_pango_fill_layout (cr, layout);
       cairo_restore (cr);