Houdini: 1. Added image_plane_functionality.py module to houdini/rendering/mantra_functionality package. This module sets or appends the helga pipeline image planes to the selected mantra ROPs. If an image_plane already exists, it will not be added twice or modified. Non standard helga pipeline image_planes are left intact. 2. Modified houdini/setup/toolbar/helga.shelf to integrate image plane adding in ui.