Fix volume interpolation in positional audio nodes

[Waridley]

Should fix #22016
Volume was interpolating from either 0 or the current volume, instead of the previous volume.

The issue I was having was with long sounds moving around the player in 3D, not with the rapid, short sounds mentioned in the linked issue. Perhaps someone should verify that this fix actually fixes that issue before closing it, but this definitely fixes my issue, and it appears to me to be the cause of the short-sound issue as well.

[Waridley]

Just downloaded Jitspoe's minimal reproduction project, and it does fix it for that as well.

[akien-mga]

Thanks for the contribution!

There are a few things that need to be changed/improved:

• PRs should be made against the master branch. Release managers are in charge of cherry-picking relevant fixes to stable branches.
• Audio clipping / static / interference on rapid intervals of sound (fixed in master) #22016 also mentions AudioStreamPlayer2D, so it's likely that the issue needs to be fixed there too. AudioStreamPlayer should also be reviewed.
• The commit log should be amended to be more explicit about what it does (see CONTRIBUTING.md), e.g.

Fix volume interpolation in AudioStreamPlayer

Fixes #37279.

[Waridley]

You're right, it needed fixed in AudioStreamPlayer2D as well.

The basic AudioStreamPlayer seems to only support fading out at the moment, not fading in, and the logic for that seems correct. Should adding fade-in support be a separate PR?

Also... Is it possible to change which branch this PR is made against without making a separate PR?

[akien-mga]

The basic AudioStreamPlayer seems to only support fading out at the moment, not fading in, and the logic for that seems correct. Should adding fade-in support be a separate PR?

That should likely be a separate PR (though we probably need to assess if it's missing by design or because it was forgotten.

Also... Is it possible to change which branch this PR is made against without making a separate PR?

Yes, though it's a bit tricky. You'd have to:

• Change the target branch by editing the PR:
  
• Cherry-pick your commit on a new branch locally, based on master
• Force push that local branch to your remote fix_22016 branch

[Waridley]

I think it tried to run the CI in between me force-pushing the new branch and me changing the base branch because I did them in the wrong order, but it should be correct now.

[akien-mga]

Indeed, the CI doesn't seem to properly support PRs changing base branch it seems. Could I ask you to do another (noop) force push to force CI to restart?

[Waridley]

Sure. Rebased onto a more recent commit anyway.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.2.