[PATCH] 8160325: Provide a public API to obtain the FXCanvas for an embedded scene.

[Alexander Nyssen]

Hi Kevin, all,

attached please find a patch that fixes JDK-8160325. The patch comprises the following changes:

- Provided static FXCanvas#getFXCanvas(Scene) method to obtain the FXCanvas instance embedding the given Scene instance.
- Added EmbeddedWindow.getHost() so the HostInterface can be retrieved.
- Added FXCanvasTest with a test method to test correct behavior of FXCanvas#getFXCanvas(Scene).
- Introduced SwtTest JUnit MethodRule to have more concise tests and ensure it is also used by SWTCursorsTest.
- Ensured SWT tests are executed using GTK2 on Linux.

I reworked the existing SWTCursorsTest while introducing FXCanvasTest to be more concise.

Regards,
Alexander