[test] Add test for IT portamento after cut and fade

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix IT portamento after cut and fade notes

IT ignores portamento after a note cut event and doesn't reset envelope
after a note fade. In compatible GXX mode, envelope is reset.

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

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