This is a follow-up after https://bugs.openjdk.org/browse/JDK-8324941
From https://github.com/openjdk/jfx/pull/1836#pullrequestreview-3055871049

 It might be useful to add two manual headless tests (or one test with two modes) -- one using Scene::snapshot and the other using Robot::getScreenCapture -- the tests would contain a scene and then save the resulting snapshot / screen capture to a PNG file, which could then be opened. Alternatively, the test(s) could be a headful Swing app that draws the captured image into a Swing JPanel for display. Since the automated system tests already exercise some of this, I think this manual test could be done separately.