Stb image loader

[riccardobl]

This PR replaces (and deprecates) AWTLoader, HDRLoader, TGALoader and the android native loader (except for webp, heic, heif), with a multiplatform java port of stb-image.

Based on : #2721

[riccardobl]

@gemini-code-assist do a new review

[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:

• Direct Download: screenshot-test-report
• Alternatively, 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

Contact @richardTingle (aka richtea) for guidance if required

[gemini-code-assist]

Code Review

This pull request introduces StbImageLoader as a unified, cross-platform image loader using the stb-image library, leading to the deprecation of AWTLoader, HDRLoader, and TGALoader. It also adds support for R16F and RG16F formats in ImageCodec. Review feedback highlights a potential thread-safety issue with the shared StbImage instance, suggests marking 8-bit luminance formats as sRGB for consistency with previous loaders, and notes a minor formatting correction.