ENH: A new filter for triangulating contours.

The vtkContourTriangulator will convert contours such as those
produced by vtkCutter, vtkContourFilter, or vtkMarchingSquares
into triangulated polygons.  It works even in cases where there
are inner contours embedded as holes in outer contours.  Before
becoming its own class, this code was part of vtkClipClosedSurface.

Change-Id: I65edf21be995688fba15bc3053a0fab956782afa
Merge topic 'contour-triangulator' into master

811d923d ENH: A new filter for triangulating contours.