code: public int addCollidable(GVRSceneObject sceneObj)
      {
          synchronized (mCollidables)
          {
              int index = mCollidables.indexOf(sceneObj);
              if (index >= 0)
              {
                  return index;
              }
              mCollidables.add(sceneObj);
              return mCollidables.size() - 1;
          }
      }
docs: Adds another scene object to pick against.
  Each frame all the colliders in the scene will be compared
  against the bounding volumes of all the collidables associated
  with this picker.
  @param sceneObj new collidable
  @return index of collidable added, this is the CursorID in the GVRPickedObject