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!