Refactor: SpotLightShadowRenderer reduce object allocations

[capdevon]

This PR refactors the SpotLightShadowRenderer class to improve code readability, consistency, and performance. Key changes include:

• Replaces hardcoded array allocation of Camera[] cameras in methods with a class-level field to avoid redundant allocations.
• Cleans up and clarifies Javadocs and comments.
• Minor improvements to method parameter usage (e.g., use of temporary vectors).
• Removes unnecessary lines and improves code formatting and spacing.

These changes do not modify the core logic or functionality but streamline the implementation and improve maintainability.

[github-actions]

🖼️ Screenshot tests have failed.

The purpose of these tests is to ensure that changes introduced in this PR don't break visual features. They are visual unit tests.

📄 Where to find the report:

• Go to the (failed run) > Summary > Artifacts > screenshot-test-report
• Download the zip and open jme3-screenshot-tests/build/reports/ScreenshotDiffReport.html

⚠️ If you didn't expect to change anything visual:
Fix your changes so the screenshot tests pass.

✅ If you did mean to change things:
Review the replacement images in jme3-screenshot-tests/build/changed-images to make sure they really are improvements and then replace and commit the replacement images at jme3-screenshot-tests/src/test/resources.

✨ If you are creating entirely new tests:
Find the new images in jme3-screenshot-tests/build/changed-images and commit the new images at jme3-screenshot-tests/src/test/resources.

Note; it is very important that the committed reference images are created on the build pipeline, locally created images are not reliable. Similarly tests will fail locally but you can look at the report to check they are "visually similar".

See https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-screenshot-tests/README.md for more information