Added static Time class
Window class updates the static delta_time field of Time class. To accomplish this I added end_frame() method to Window class. Not the best solution, because now it is necessary to call this method at the end of the game loop if you want to update delta_time variable (see FlappyBird or 3D cube examples). I think, it could be fixed with Application layer. For now, we can stay with this solution
Added static Time class Window class updates the static delta_time field of Time class. To accomplish this I added end_frame() method to Window class. Not the best solution, because now it is necessary to call this method at the end of the game loop if you want to update delta_time variable (see FlappyBird or 3D cube examples). I think, it could be fixed with Application layer. For now, we can stay with this solution