Run windows examples in linux

[riccardobl]

Since this is the year of the linux desktop, this PR adds a runExamplesWithProton task

./gradlew runExamplesWithProton

or

./gradlew runExamplesWithProton -Pexample=jme3test.light.pbr.TestPBRSimple

that runs the examples through umu-launcher inside proton.

With this we can test jme for windows from any linux machine.

[gemini-code-assist]

Code Review

This pull request introduces new Gradle tasks to download the umu-launcher and a Windows JDK, enabling the execution of jMonkeyEngine examples within a Proton environment. The review feedback highlights several opportunities to make the build script more idiomatic and robust, including declaring input properties for proper incremental builds, using 'ant.get' for more reliable file downloads, and leveraging 'fileTree' for locating binaries. There is also a suggestion to simplify the classpath construction by removing redundant dependency resolutions.

[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