[vtkusers] Mesh visualization

[Will Schroeder]

Hi Folks-

In response to Patrick Lagace's request I've added a flag in vtkMapper to address the issue of
rendering coincident points/lines on top of polygons. I did not use Patrick's glPolygonOffset()
call (which works splendidly with lines) but rather I used the glDepthRange() to adjust z-buffer 
mapping. (Bob Obara at Simmetrix actually was the one who implemented it initially.) The reason 
that I did not use the glPolygonOffset() approach is because it did not address the point/line 
coincident problem, that is, you might want to show points on top of lines on top of polygons. 

Anyway, the flag is ResolveCoincidentPrimitives (and there is a global flag 
GlobalResolveCoincidentPrimitives) in vtkMapper. (I don't like the name, if there are other
suggestions I'd like to hear them.) It works for OpenGL currently. The name could change
in the future, and we might even consider making the rendering process always do it. 

I'd appreciate giving it a try and seeing what you think. It's in CVS or available tomorrow in
the nightly release.

Will