Add envelope loop and sustain point quirk

FT2 and IT envelopes behave in a different way regarding loops, sustain and
release. When the sustain point is at the end of the envelope loop end and
the key is released, FT2 escapes the loop while IT runs another iteration.
See EnvLoops.xm in the OpenMPT test cases.

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Set mixer pan to 100% on OpenMPT tests

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

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

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Move OpenMPT XM test cases

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT XM test cases documentation

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Rename OpenMPT XM tests to include the format name

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add OpenMPT S3M test case files

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

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