Player engine tweaks.

As DigiBooster 3 got ProTracker module import, the player has been tweaked to
get better compatibility with ProTracker:
- Sample offset is not reset at start of row.
- Sample offset is not reset at instrument trigger.
- If instrument is specified on a position, and it differs from the current
  one, it is changed, regardless of note or porta to note effects.
- If instrument is specified on a position, sample offset is zeroed.