fix: Combine polyline interpolation breaks annotation state data

[wayfarer3130]

Context

If the list returned by getAnnotations is modified, it directly affects the underlying data object, breaking further interpolation.

Also fixes [#1091]

Changes & Results

Fixed the interpolation retrieve check to take a copy of the data returned from getAnnotations.

Testing

Create two spline interpolations, one slice apart. Combine an annotation. The original annotations shouldn't go blank.

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

• [] I have run the yarn build:update-api to update the API documentation, and have
  committed the changes to this PR. (Read more here https://www.cornerstonejs.org/docs/contribute/update-api)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] "OS:
• [] "Node version:
• [] "Browser:

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[wayfarer3130]

@lscoder - I've responded to both your comments. If that is ok, could you approve it and I will merge? The defect is currently corrupting internal annotation state, so I'd like to get it this far.