Fixed parallax bugs

Fixed these bugs in parallax :point_right:  https://trello.com/c/syOn3PuP/279-parallax-hero-banner-is-borked-when-user-loads-page-with-it-already-in-viewport-and-when-user-resizes-viewport

I've tried to make a gif... but with no luck.. these bugs can't be captured in a gif..