fix examples/util/src/outputVtk.cpp for octree examples
Change the polygon selection policy and the definition of boundary ID as follows:
  1. All polygon groups are searched by Polylib::search_polygons method.
  2. The polygons whose exid between 1 and 255 are used for intersection calculations.
  3. Each polygon's exid is adopted as a boundary ID.
Merge pull request #2 from szks/use-exid

Use exid