Add `SpriteAlphaLens`

[johanhelsing]

Sometimes it's convenient not to overwrite the color of a sprite. If they have conflicting animations or are controlled by other systems for instance.

Makes it a lot simpler to create simple "fade out" animations:

                    .insert(Animator::new(Tween::new(
                        Linear,
                        Duration::from_secs_f32(0.3),
                        SpriteAlphaLens { start: 1., end: 0. },
                    )))