* spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh,
spot/twaalgos/translate.cc: Introduce a gen-reduce-parity option and
use it on sub-automata built by ltl-split.
* bin/spot-x.cc: Document it.
* tests/core/ltl2tgba2.test: Add test case reported by Juraj Major.