game: Remove unused function confine_int()
game: Rename function update_mouse() to update_pointer()
game: Use constants to name layers
game: Add time-out to the game
game: Add function for drawing numbers
game: Add drawing of timer showing remaining seconds
game: Add drawing of score
world: Add WORLD_SIZE_* constants
sprite: Add sprite_set_animation() function

When a sprite is initialized, its animation is now set to NULL.  To
start a new animation, we call sprite_set_animation() and specify the
point in time when the animation has been started.
sprite: Remove v_x and v_y fields from sprite structure

The velocity of an actor is a world-level concept, not a display-level
concept.  Updating the display position of a sprite automatically is
also not really useful because it is not synchronized with the world
coordinates of the corresponding actor.
duck: Make sprite classes public
compat: Add frand() function
main: Initialize random number generator
world: Add screen-to-world transformations
game: Add creation, updating, and destruction of ducks