Pass button press events to guest side if gaining focus in seamless mode

[FeralChild64]

Description

Provide a hint to the SDL library, resulting in a better seamless mouse integration behavior; see the Manual testing chapter for the use case.

Thanks to @Torinde for signaling the problem.

Related issues

#3290

Manual testing

• set mouse_capture to seamless, run DOSBox in a window
• launch DOS Navigator, Norton Commander, or other program having menus
• click outside of of the DOSBox window, so that it loses the focus
• click on the DOS navigator menu - even the first click, when DOSBox window has no focus yet, should be now passed to the guest software (before the PR the results were OS-dependent; on Linux with X11 and KDE the behavior was random, on Windows 11 the first mouse click was never propagated to guest)

Checklist

I have:

• followed the project's contributing guidelines and code of conduct.
• performed a self-review of my code.
• commented on the particularly hard-to-understand areas of my code.
• split my work into well-defined, bisectable commits, and I named my commits well.
• applied the appropriate labels (bug, enhancement, refactoring, documentation, etc.)
• checked that all my commits can be built.
• confirmed that my code does not cause performance regressions (e.g., by running the Quake benchmark).
• added unit tests where applicable to prove the correctness of my code and to avoid future regressions.
• made corresponding changes to the documentation or the website according to the documentation guidelines.
• locally verified my website or documentation changes.

[kcgen]

Nice find, @FeralChild64 !

[Torinde]

Tested the fix, it works, thanks @FeralChild64!