Improve /firefox/os/ animation performance. Bug 911674.

- Update intro bgs, phone screen, adaptive bgs, adaptive
  feature points, & adaptive sprites.

- Performance in A-grade browsers seems far better (based
  on Fx repaints and Chrome frames).

- IE 9 and below get a slightly degraded experience. Can
  be improved if necessary.

- Fix IE8 bug where birthday bg would never show up in
  top fading section.