A float driver that returns the playback speed of an
animation on the source, as a multiplier. 1 is the normal speed, and an animation the
client plays at double speed returns 2.
The source must be a unit. The driver returns 1 without one, when the name is not in
the animation graph of the unit, and whenever the animation is not playing.
Shipped data uses it for one thing: the play speed of a
PersistentVfxData effect, so that the effect keeps pace
with the animation it belongs to.
