fix(updateIndex): getNumberOfSlices is defined when used with a 3D viewport

[Ceciliadrc]

Context

This PR fix getNumberOfSlices that is not defined in 3D viewports.

Changes & Results

Testing

Use a 3D viewport to test it

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.)

Public Documentation Updates

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

Tested Environment

• OS: Windows 11
• Node version: 20.16.0
• Browser: Chrome

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[salimkanoun]

(From Pixilib team)