RFR: 8359266: Delete the usage of AppContext in the GraphicsDevice

[Alexander Zvegintsev]

On Mon, 16 Jun 2025 06:17:32 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

> 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](https://github.com/openjdk/jdk/blob/23e1e2ff4a4a75ec268c7925fb98d6b96a01bbcf/src/java.desktop/macosx/classes/sun/awt/CGraphicsDevice.java#L226)).
>  - 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.

Marked as reviewed by azvegint (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/25818#pullrequestreview-2966821387