Mixer refactor

[M374LX]

Some refactoring of the Mixer class.

74fc3f2 moves the resizing of PlayHandles from Song to Mixer, since Mixer is the class that contains the PlayHandles.

[PhysSong]

I'm not sure why we need 74fc3f2. Mixer holds PlayHandles, but the tempo is a property of Song. The other commits look fine to me.

[M374LX]

74fc3f2 moves to Mixer only the part of the tempo change process that resizes the PlayHandles.

[JohannesLorenz]

@M374LX Is this ready for review, or are you working on this?

[BaraMGB]

Should we close pull requests because of the opener is not answer for a while?

[Reflexe]

@BaraMGB IDK, What's the reason behind leaving abandoned WIP pull requests? it makes finding active PRs more difficult.

[JohannesLorenz]

In theory you could just filter the "response required" out, and add the list of PRs with that filter to your bookmarks. In practice, I'm also too lazy to do that.

[M374LX]

@JohannesLorenz
It is ready for review. I have updated the "mixer-refactor" branch.

I am sorry for being busy lately and unable to give attention to my own PRs.

Can this PR be reopened now?

[JohannesLorenz]

The other commits look fine to me.

If I read that right, @PhysSong has already confirmed that this can be merged?

[PhysSong]

I'm still against 74fc3f2, but it's just an opinion. Maybe we can revert some formatting changes in order to reduce the amount of changes while keeping essential contents. For 2c695e2, I think a range-based for loop is better than a while loop.
There are minor conflicts with #4878, but they aren't hard to resolve.

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[M374LX]

Rebased.

Using a range-based for loop, as suggested by @PhysSong.

Should I undo 74fc3f2 (fcb0ce7 after the rebase)?

Other than that, anything preventing the merge?

[Veratil]

At quick glance, many style updates need to be done.

[PhysSong]

Should I undo 74fc3f2(fcb0ce7 after the rebase)?

IMO yes because Mixer is for rendering, routing and collecting audio signals.

[M374LX]

Reverted.

@Veratil We can deal with style updates later.

[PhysSong]

@JohannesLorenz Could you review this or remove the review request?

[JohannesLorenz]

I removed my review requests, since @PhysSong has already approved the PR.