Game elements visibility in actual project

Somewhat related to #140, this fork is my attempt to get the AsteroidBlaster game from the tutorial running in a standalone macOS app:

https://github.com/lcs-rgordon/ImagineEngine

I've tried to keep my commits atomic so that it's easy to see what I've done:

https://github.com/lcs-rgordon/ImagineEngine/commits/master

Doing my best to follow @JohnSundell advice from Twitter I'm using the AppDelegate to create a GameWindowController instance to launch the game:

https://mobile.twitter.com/johnsundell/status/951739402845384704

When the game window opens, the dark blue background is visible, and I've confirmed using the debugger that ImagineEngine is doing it's thing (for example, I can see that asteroids are spawning every two seconds).

However, the window only shows a dark blue background.

I'm betting I've missed something, but I've opened this issue in case there is something with ImagineEngine that could be fixed.

Forgive me if I'm not reporting this issue correctly, I'm a relative newbie at contributing to open source projects via GitHub.