* spot/twaalgos/degen.hh, spot/twaalgos/degen.cc: Adjust
degeneralize() and degeneralize_tba() to work on generalized-co-Büchi.
* NEWS: Mention this.
* spot/twaalgos/cobuchi.hh, spot/twaalgos/cobuchi.cc (to_nca): Use
degeneralization on generalized-co-Büchi.
* spot/twaalgos/postproc.cc: Use degeneralization for generalized
co-Büchi as well.
* bin/autfilt.cc: Improve chain products of co-Büchi automata by using
generalization if too many colors are needed.
* tests/core/prodchain.test, tests/python/pdegen.py: Add test cases.