8359266: Delete the usage of AppContext in the GraphicsDevice

[mrserb]

This PR removes the usage of AppContext from the GraphicsDevice class. The original use case was to store the full-screen window in some AppContext, which was necessary in the context of plugin/appletviewer environments. However, there is now effectively only one main AppContext, so this indirection can be eliminated.

Notes: GraphicsDevice provides two methods for handling full-screen windows:

• setFullScreenWindow() is considered thread-safe, as all platform-specific implementations override it with synchronized methods (example).
• getFullScreenWindow() previously relied on AppContext for thread-safety. After removing AppContext, this method is now made thread-safe by declaring the fullScreenWindow field as volatile. This change has been validated by an added test.

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Issue

• JDK-8359266: Delete the usage of AppContext in the GraphicsDevice (Enhancement - P3)

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