animatedstyle: don't share styleanimations

animatedstyle: don't share styleanimations

Because of our port of css animation and css transition to
progress tracker, we should not think of animated styles as
immutable objects that can map any timestamp to css values.
Rather, timestamps can correspond to different values depending
on the value of GTK_SLOWDOWN over the course of the animation.

To keep animated styles and style animations totally immutable,
we will not share styleanimations between animatedstyles, and
make a new copy of a styleanimation for each timestamp.