to_parity: Add an option to force degeneralization

* spot/twaalgos/toparity.cc, spot/twaalgos/toparity.hh:
Don't try to run the algorithm without degeneralization
with the option force_degen.

spot/twaalgos/toparity.cc

@@ -1606,6 +1606,7 @@ run()
             max_states_sub_car =
                 10000 + nb_states_deg - 1;
         }
+        if (!options.force_degen || !has_degeneralized)
             nb_states_sub =
                 build_scc(sub_automaton, scc, state2car_sub, car2num_sub,
                           algo_sub, max_states_sub_car);

spot/twaalgos/toparity.hh

@@ -45,6 +45,9 @@ namespace spot
     /// degeneralization to remove occurrences of acceptance
     /// subformulas such as `Fin(x) | Fin(y)` or `Inf(x) & Inf(y)`.
     bool partial_degen = true;
+    /// If \c force_degen is false, to_parity will checks if we can
+    /// get a better result if we don't apply partial_degeneralize.
+    bool force_degen = true;
     /// If \c scc_acc_clean is true, to_parity() will ignore colors
     /// no occoring in an SCC while processing this SCC.
     bool acc_clean = true;