[Ultra Tracker] Tone portamento does not continue between patterns.

See discussion in #40: Ultra Tracker tone portamento is currently implemented by unrolling the tone portamento effect. This works within a single pattern, but does *not* work when the tone portamento continues *between* patterns. The only fix for this is to implement a true continuous effect.

https://github.com/sezero/mikmod/files/7221268/PORTA.ULT.zip