Limit filter envelope value

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Update filter cutoff range

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix IT filter envelope carry after envelope end

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT filter-nna and flt-env-carry test cases

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>