feat(viewport): add getScale function

[julianlinaress]

Context

Adding a getScale function would be useful to compare original and rendered image size.

Changes & Results

Added a getScale method to the Viewport class to calculate the scaling ratio between the rendered image size and its original dimensions. This is useful for understanding how an image is being displayed relative to its original size.
Makes it easy to verify correct image scaling behavior, helps troubleshoot display issues and identifies unexpected non-uniform scaling that could distort the image

Testing

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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