Reset background channel if note ends

If channel is a background channel we can safely reset it because there
will be no tone portamento effect afterwards.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Add virtual channel saturation test

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Comment out unused code in YM3812 emulator

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