Backport vtkdelaunay2d fixes to release-6.3
Retrieve 2 fixes from master to 6.x release for vtkDelaunay2D.
MR 716 @jpouderoux : Fix a bad index issue that make constrained delaunay2d crash
MR 762 @Toki : Fix FindTriangle
Retrieve 2 fixes from master to 6.x release for vtkDelaunay2D.
MR 716 @jpouderoux : Fix a bad index issue that make constrained delaunay2d crash
MR 762 @Toki : Fix FindTriangle