Sets the playback speed factor of the specified animation.
This action applies to all entities which expose the iAnimationMixer interface. This is notably the case of the Character entity.
When AnimStartSpeed and AnimEndSpeed are different values the speed value at a given time is the interpolation of the two values relative to the current time at which the action is currently set. So if the action's time scrubber is in the middle of its time block the speed value will be right in the middle between AnimStartSpeed and AnimEndSpeed.