Repaint Fixes When Scrolling

Heya guys!

Here's my 2nd attempt at this. Thanks again for being so patient with my first try.

Ideally, `translateZ(0)` would work the best in the spots that I've added the vendor-prefix mixin of `.translate3d()` as it has IE9 support. Based on feedback from my first PR, I avoided adding a mixin specifically for `translateZ`.

The translate3d fix for `.modal` is tricky. It effectively fixes repaints on scroll.. but it might be problematic for IE  8-9.

Thanks!