Fix loss of precision in tone portamento

Our periods are defined as doubles, but when performing a tone portamento
the target portamento was stored as an integer. Change that to double so
we can slide to correct periods.

Reported by Misty De Meo, check jazz jackrabbit 2 - prerelease - medivo.it
order 12, channel 10.

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

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
[test] Update tests to reflect portamento precision fix

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