PhysicsShapeGroup2D.Add() incorrectly merges shape start vertex.

Reported on the forums here: https://forum.unity.com/threads/physicsshapegroup2d-generates-argumentoutofrangeexception.1518040/

When using "PhysicsShapeGroup2D.Add()" to add a PhysicsShapeGroup2D to an existing shape group, the added shapes have invalid [vertexStartIndex|https://docs.unity3d.com/ScriptReference/PhysicsShape2D-vertexStartIndex.html.]