Provides a display option, zoom-to-fit, that enables scaling of the
display when full-screen mode is enabled.

Also ensures that the corresponding menu item is marked as enabled when
the option is set to on.

Signed-off-by: Carwyn Ellis <carwynellis@gmail.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20231027154920.80626-2-carwynellis@gmail.com>