Disable virtual channels in IT sample mode

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix portamento after note end in IT sample mode

In IT sample mode, retrig the sample if a tone portamento is used after
a single-shot sample and the sample has ended. See UNATCOReturn_Music.it
pos 0x27 channel 13, sample 12.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add Stephen Leary to credits file

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fixes for ABK loader including memory issues

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