| shapeCapacity | The initial capacity of the PhysicsShape2D list used to contain the shapes. |
| vertexCapacity | The initial capacity of the vertices list used to contain the shape geometry. |
Initializes and returns an instance of PhysicsShapeGroup2D. The shape group will be empty and ready for use by Collider2D.GetShapes, Rigidbody2D.GetShapes or manually adding shapes.
Both the shapeCapacity and vertexCapacity should be set high enough so that new allocations are not required and therefore no garbage collection work is produced.