Comment out unused events in mod and st3 readers

Fade and cut events are not used in mod and st3 readers, so comment them
out (but keep the commented out code there for reference).

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Set ST3 quirks and event reader for Poly Tracker

Poly Tracker seems to be heavily ST3-influenced, so use ST3 quirks and
event reader for PTM modules.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Event reading code cleanup

Remove comments and tests for conditions that never happen.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Update portamento envelope reset tests

FT2 resets envelopes only when a new instrument is set. IT resets envelopes
when not using tone portamento. In compatible GXX mode, reset on tone
portamento only when an instrument is set.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix XM and IT envelope resets

FT2 resets envelopes only when a new instrument is set. IT resets envelopes
when not using tone portamento. In compatible GXX mode, reset on portamento
only when an instrument is set.

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

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