Use mid channel to find peaks but unwrap phases in left and right

[Paul-Licameli]

Resolves: #5603

(short description of the changes and the motivation to make the changes)

• I signed CLA
• The title of the pull request describes an issue it addresses
• If changes are extensive, then there is a sequence of easily reviewable commits
• Each commit's message describes its purpose and effects
• There are no behavior changes unnecessary for the stated purpose of the PR

Recommended:

• Each commit compiles and runs on my machine without known undesirable changes of behavior

[LWinterberg]

this build can invert phase polarity during processing

waveform:

after render:

[Paul-Licameli]

Curious... maybe there is something "obvious"

[Paul-Licameli]

Can you share the example file?

Do you observe any difference if you vary the amount of initial hidden waveform?