Add ISceneNodeAnimator::setStartTime/getStartTime to allow resetting movement animators. This should work for:
CSceneNodeAnimatorFlyStraight, CSceneNodeAnimatorFlyCircle, CSceneNodeAnimatorFollowSpline, CSceneNodeAnimatorTexture.
It's also implemented for CSceneNodeAnimatorRotation, but that one currently resets the StartTime constantly (I don't think 
it has to, but I have no tests written for animators currently so can't change that for now).


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4675 dfc29bdd-3216-0410-991c-e03cc46cb475