/* --- *
S c a l e   C o n s t r a i n t
--- 
Relates two variables by the linear scaling relationship: "v2 =
(v1 * scale) + offset". Either v1 or v2 may be changed to maintain
this relationship but the scale factor and offset are considered
read-only.
function ScaleConstraint(src, scale, offset, dest, strength) {
  this.direction = Direction.NONE;
  this.scale = scale;
  this.offset = offset;
  ScaleConstraint.superConstructor.call(this, src, dest, strength);
}