Fix credits not appearing in Play Mode

[j9liu]

Fixes #564. Definitely try to break this while testing and find any edge cases. Things I've tested so far:

• Switching quickly between Play and Edit modes
• Opening multiple simultaneous scene tabs
• Switching between Game and Scene views during Play Mode
• Adding and removing scenes

[kring]

Thanks @j9liu, this is much better! I still see one problem where the Cesium ion logo sometimes disappears. I can reproduce it pretty reliably like this:

1. Open the Samples project (I'm in Unity 2022.3.41f1, but hopefully it doesn't matter)
2. Open a Cesium level if one isn't already opened. The Cesium ion logo will appear in the Scene view as expected.
3. Open a different Cesium level. The Cesium ion logo will be missing:
   
4. Press play. The Cesium ion logo appears in the Game view.
5. Stop play. The Cesium ion shows up in the Scene view now, too. (though it may take a couple of seconds to appear)

[j9liu]

Thanks for the catch @kring ... looks like I forgot to test some common cases in favor of the obscure ones 🤪

[j9liu]

For some reason the HandleClosingSceneView was never being called, so I'm moving the editor state management into an Editor-only class with the [InitializeOnLoad] attribute. That results in more reliable behavior.

[j9liu]

@kring ready for another look! If you want to test runtime scene changes as well, I made these modifications to CesiumSampleScene.cs:

    void Update()
    {
#if ENABLE_INPUT_SYSTEM
        bool pressed1 =
            Keyboard.current.digit1Key.wasPressedThisFrame || Keyboard.current.numpad1Key.wasPressedThisFrame;
        bool pressed2 =
            Keyboard.current.digit2Key.wasPressedThisFrame || Keyboard.current.numpad2Key.wasPressedThisFrame;
        bool shiftPressed = Keyboard.current.shiftKey.isPressed;
#else
        bool pressed1 = Input.GetKeyDown(KeyCode.Alpha1) || Input.GetKeyDown(KeyCode.Keypad1);
        bool pressed2 = Input.GetKeyDown(KeyCode.Alpha2) || Input.GetKeyDown(KeyCode.Keypad2);
        bool shiftPressed = Input.GetKeyDown(KeyCode.LeftShift) || Input.GetKeyDown(KeyCode.RightShift);
#endif

        LoadSceneMode mode = shiftPressed ? LoadSceneMode.Additive : LoadSceneMode.Single;
        if (pressed2)// && EditorWindow.focusedWindow == SceneView.lastActiveSceneView)
        {
            SceneManager.LoadScene("02_CesiumMelbourne", mode);
            //ResetSceneView();
        }
        else if (pressed1)
        {
            SceneManager.LoadScene("01_CesiumWorld", mode);
        }
    }

[kring]

Thanks @j9liu!