SpringArmMargin Property

When the collision check is made, a candidate length for the SpringArm is given.

The margin is then subtracted to this length and the translation is applied to the child objects of the SpringArm.

This margin is useful for when the SpringArm has a Camera as a child node: without the margin, the Camera would be placed on the exact point of collision, while with the margin the Camera would be placed close to the point of collision.