Add quirk for S3M-style square vibrato waveform

The square waveform for vibrato in S3M/IT range from 0 to 255, while the
"standard" MOD/XM square waveform range from -255 to 255.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix fine and extra fine portamento handling

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add linear period vibrato handling

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Remove unused synth layer method

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

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