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Gets the animated value, given the elapsed fraction of the animation (interpolated by the
animation's interpolator) and the evaluator used to calculate in-between values. This
function maps the input fraction to the appropriate keyframe interval and a fraction
between them and returns the interpolated value. Note that the input fraction may fall
outside the [0-1] bounds, if the animation's interpolator made that happen (e.g., a
spring interpolation that might send the fraction past 1.0). We handle this situation by
just using the two keyframes at the appropriate end when the value is outside those bounds.