add skeleton Tunnel mode, no content yet

src/dwss.cc

@@ -34,8 +34,9 @@ static Mode *getMode(GnomeScreensaver & gs)
 {
 #if 0
     return new Starfield(gs);
-#endif
     return new Spin();
+#endif
+    return new Tunnel();
 }
 
 int main (int argc, char *argv[])