stylecontext: Add an animating framework
The design principles were: - synchronized If multiple style contexts are animating, they should all do an animation step at the same time. - degrades well Even when there's thousands of style contexts all animating at the same time, the animation steps don't starve the CPU. This is achieved by making sure the timeout is really fast. It just sets a bunch of flags. - no hidden bottlenecks Turning animatability on or off on a style context is O(1). So far it is unused.
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