Fix XM portamento with volume column portamento

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix XM fadeout on keyoff with instrument

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT NoteOff2.xm test case

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