[vtkusers] how to draw overlays on a image

[dean.inglis at camris.ca]

if you are rendering planar polygonal wireframe objects
(lines, etc.) over a vtkImageActor, for example,
make sure the point coordinates share the same
in plane location (e.g., z coordinate) of the object you are
rendering over and then set the 
vtkPolyDataMapper of your line actor with a call
to the static member function:

vtkPolyDataMapper::SetResolveCoincidentTopolgyToPolygonOffset()

If you want to actually draw on an image, look at
class vtkImageCanvasSource2D.

Dean