On at least iOS 26 using a temporary window frame is unreliable. This
change tries to use an existing window for any non-standalone app. It
also updates the details on any changes, such as when the device
orientation changes.
This also fixes an issue where endBackgroundTask wasn't guaranteed to be
called after suspended() completed. According to the docs,
endBackgroundTask must be called after the task completes.
initWithWindowScene appears to create a window with no area if the
provided scene is null. As a fallback, creating a window without an
associated scene still returns usable bounds.
Previously, the UnitTestRunner example in the DemoRunner would fail on
any tests that included ScopedJuceInitialiser_GUI, as DeletedAtShutdown
objects would be destroyed too early.
