Change offsetVolume function

[tk26eng]

What this patch does to fix the issue.

This PR will improve the performance.
And this PR also make the logic simpler a little.

Link to any relevant issues or pull requests.

Fix #1164

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• Readability Approval for C++ from tkng, tsawada, primenumber

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[iizukak]

OA

@primenumber Can you review as readability reviewer?

[primenumber]

RA

[tk26eng]

/ready

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[iizukak]

/ready

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