Related to issue #298.
* spot/twaalgos/determinize.cc: Recognize braces that are temporary
to avoid emitting colors when they become empty.
* tests/python/298.py: New file, showing a reduction of colors.
* tests/Makefile.am: Add it.
* tests/core/ltlsynt.test: Adjust expected output (now smaller).
* tests/core/genltl.test: Adjust one expected output (now larger).
* NEWS: Mention the issue.
