Finds the closest loop from the OdDbMPolygon within the pick box around the specified 3D point. 

Index of the closest loop.

• Returns -1 if there is no loop in the pick box.
• The pick box is created around the input point. The size of the pick box is specified by the system variable PICKBOX.
• If two loops are at the same distance from the point, the function finds a loop with a lower index.
• If two loops are within the pick box, the function finds a loop that is closer to the worldPt point.