Hold instrument behavior inconsistent between tracker and NSF export

[Threxx11]

The && (hold instrument) command is meant to allow changing pitch in the tracker without resetting or retriggering the previous instrument envelope. As demonstrated in the linked module, when the && command is used, the pitch changes while allowing the envelopes of the instrument (volume and duty, in this case) to persist.

In the NSF export, while the duty envelope is properly maintained, the volume envelope is reset to the volume that the channel was previously set to globally, rather than continuing the given volume envelope.

Module: https://cdn.discordapp.com/attachments/219258602450452486/898301011226132550/hold_instrument.0cc NSF: https://cdn.discordapp.com/attachments/219258602450452486/898301162393047141/hold_instrument.nsf

[Gumball2415]

apparent duplicate of #63?

[Gumball2415]

apparently not a duplicate of #63, as that issue pertains to bugs inside the tracker, not the export

this is one of those features that eventually need to be fixed/added to the NSF driver, since both in-tracker and NSF driver development haven't been regularly maintained to feature-parity for a long time.