minimize_wdba() failed to fully minimize some automata.

* src/tgbaalgos/minimize.cc (minimize_wdba): Fix the Löding
algorithm to use colors.  The previous implementation was an
incorrect approximation.
* src/tgbatest/wdba2.test: New file showing two equivalent
formulas that were minimized in automata with different sizes.
* src/tgbatest/Makefile.am: Add it.

src/tgbatest/wdba2.test

@@ -0,0 +1,38 @@
+#!/bin/sh
+# Copyright (C) 2012 Laboratoire de Recherche et Développement
+# de l'Epita (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Spot; see the file COPYING.  If not, write to the Free
+# Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+# 02111-1307, USA.
+
+. ./defs
+
+set -e
+
+cat >expected <<EOF
+sub trans.: 16
+transitions: 8
+states: 4
+EOF
+
+# These two equivalent formulae used to produce
+# minimized automata of different sizes...
+run 0 ../ltl2tgba -Rm -kt 'a | X(Gd|Fa)' > out
+run 0 ../ltl2tgba -Rm -kt 'Fa | XGd' > out2
+
+cmp out expected
+cmp out2 expected