Fix Kicker's release stage

[michaelgregorius]

When applying its release stage Kicker did not take the frames before the release into account but instead always applied the release to the full buffer. This potentially lead to a jump in the attenuation values instead of a clean linear decay.

Fixes #7225.

[zonkmachine]

I've tested by exporting kick tracks at various sample rates and different buffer sizes and the issue is fixed. The code looks fine but perhaps someone more knowledgeable should review it.

[michaelgregorius]

[...] The code looks fine but perhaps someone more knowledgeable should review it.

@zonkmachine, can you propose someone?

[Veratil]

@LostRobotMusic is our resident audio algorithm expert

[LostRobotMusic]

Normally I'd complain about the use of linear interpolation, but to my understanding this is for implementing the release in LMMS's default instrument envelope which is strictly linear currently (unfortunately).

LMMS's release code has always been wonky but this code appears to be correct. I haven't tested the PR directly, but it looks like zonkmachine already has.

[michaelgregorius]

Normally I'd complain about the use of linear interpolation, but to my understanding this is for implementing the release in LMMS's default instrument envelope which is strictly linear currently (unfortunately).

Thanks for the additional review @LostRobotMusic! This merely fixes the linear release of the Kicker instrument which was always intended to be linear but which had a bug (see #7225).

Adding exponential envelopes would be a whole different task.