Change step function conditionals to not reference pageYOffset

[jljorgenson18]

Referencing pageYOffset requires layout calculations (https://gist.github.com/paulirish/5d52fb081b3570c81e3a). A better approach would be to see how close the result of the easing function is close to '1.' We can add a percentage buffer to handle rounding errors which should stop the need to reference pageYOffset.