active GPU acceleration for iOs 6 device using webkit

Hi YUI Team,

I experimented a bug with scrollview, when it used on UIWebView, for iOs 6. On the first scrollTo(0,1), the page was "blinking" for a very short time (like 100ms).

I dive a little, and i found that Apple desactivate the hardware-acceleration with 3d transform, see those links : 
- http://developer.apple.com/library/ios/#releasenotes/General/RN-iOSSDK-6_0/_index.html#//apple_ref/doc/uid/TP40012166-CH1-SW19
- http://indiegamr.com/ios6-html-hardware-acceleration-changes-and-how-to-fix-them/

In iOs 6, the trick to active hardware-acceleration, is now to use "webkit-perspective:1000" and "-webkit-backface-visibility:hidden". So I implemented it.

The "blinking" bug is now fixed (test on iPad 1, 2 and retina, on iPhone 4 and 5, all running iOs6), and the page is hardware-accelerated.


By the way : Nice "Scrollview Dive deep" at YUIConf2012, very useful !


Julien