Fix slow loading

[Mia75owo]

This is related to: #70
This is a possible fix for this issue. I don't really like how it is implemented and it isn't really good code
but i wanted to show how my idea of how to fix it.
The loading screen may be not as smooth as the implementation before when running on a very slow device,
but i don't think it's that much of an issue.
Maybe someone can implement this in a better/cleaner way, feel free to send some feedback :)

[drfiemost]

I don't really like how it is implemented and it isn't really good code but i wanted to show how my idea of how to fix it.

Better mark this as draft then.

On a second tought I'm mostly annoyed by loading times during debug sessions, but then I can use the --novsync flag for a speed-up. For a normal play I think a few seconds wait between levels is tolerable.

[Mia75owo]

True, this mostly annoyed me too during debugging, I think I will add a flag to start directly into a level soon :) I will try to clean up this code later. I thought about running the loading bar on a second thread but this would make it way harder to port so I will try to come up with another solution.