world: Add more screen-to-world transformations
game: Swallow last key-press event
duck: Add hit test
game: Remove unused variable
game: Rename variable
game: Adapt parameters to make the game more challenging
game: Return score value to main() and print it after finishing the game
game: Put speed parameters in constants
game: Add hit test and update score

For now, we simply delete any ducks that have been hit.