An animation that displays an image fading from one alpha level to another in specified
increments over time. The animation is finished when the specified target alpha is reached.
Constructs a fade animation.
bounds our bounds.
alpha the starting alpha.
step the alpha amount to step by each millisecond.
target the target alpha level.
public void tick (long timestamp)
Here is where derived animations actually render their image.
This handles the main work of fading.