Blender Git Loki

Copy Scale: support raising the copied scale to an arbitrary power.

Since scale is multiplicative, the appropriate way to partially copy
it is to use power. However, the influence slider of constraints uses
linear interpolation. Thus, there is no way to correctly split scale
via constraints without adding this feature.

In addition, this allows inverting scale by using negative powers,
fulfilling the function of Copy Rotation's Invert checkboxes.