Correct and augment the behavior of vtkPolygonBuilder

This fix is in reference to bug report 0015582. vtkPolygonBuilder manifested two problems: triangles that were not adjacent to any triangles that were previously added to the builder were discarded, and polygons with internal vertices resulted in overlapping cells (the class was explicitly not designed to handle polygons with internal vertices). The new version fixes these two issues, and a test has been added to check these issues as well. Also, in cases where a single cell is cut into multiple cells, it is necessary to return disjoint polygon elements in vtkPolygonBuilder. A test provided by Andreas Buykx has been included that tests the ability of vtkPolygonBuilder to construct multiple disjoint polygons.