Should allow for the forcing of 2d transforms

[jpcody]

When Webkit uses hardware-accelerated animations on elements containing text, the text is rendered to an image in order to animate more efficiently. (I believe this is the cause, but the only evidence I have is cursory Googling.) You can see the difference in 2d and 3d in this fiddle.

For this reason, it'd be nice to be able to force an animation to 2d, even in 3d-supporting browsers. One example is 3d-animating an element, and binding the transition's end to switch to a 2d transform. I've hacked this together in a way that's not worthy of a pull request, which you can see in this gist.

Essentially, it's another CSS hook for use3d that defaults to true. Unfortunately, it's cached, so if you declare it false once, you need to declare it true elsewhere. And you need to declare it as the first property in the list. So yeah, my implementation sucks, but I'd love to hear your thoughts on this feature.