Don't retrig IT note in portamento with same sample

IT portamento retrigs are based on samples and not instruments. It retrigs
with different samples in the same instrument, and doesn't retrig with the
same sample in different instruments.

We should rewrite the IT event reader based on samples instead of instruments
to simplify its logic.

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT gxxtest.it test case

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