Avoid updating all animation effects on single animation mutation

[alancutter]

This change avoids applying the effects of all Animations when a single Animation's state is modified (e.g. setting currentTime).
This is particularly effective

Performance numbers on Firefox Nightly:
500 calls to element.animate(null, 1000): 1600ms down to 170ms.
The 500 Individual Stars demo (rewritten to use element.animate()): 8.5-9.5s down to 4.5-5s
Test case in #472: 2.5-4s down to 30-50ms

[ewilligers]

lgtm

[alancutter]

I discovered afterwards that the quoted performance numbers in the description are a bit bogus. Turns out Firefox's performance is heavily influenced by the developer console being open which I was not accounting for.