Bugfixes + Optimize

[einarf]

• Upgrade arcade to dev branch (see requirements.txt). This will speed up physics_engine.update() 200-300 times.
• Removed the threading in on_update that causes context issues
• The start menu buttons was still active when the game had started causing the user to click the menu buttons by accident
• Changed the drawing style from smooth to pixellated

The performance boost is temporary and when the new PhysicsEnginePlatformer is merged into arcade you need another small change.