Improve the shadow maps in VTK
This patch changes how the shadow maps work so that less shadow code is in the mappers. Now the mappers get a key that contains the shadow map pass and they let that pass handle setting up shader code and uniforms. Also changed the shadow map algorithm slightly. The old approach did a 1/3 weighting of pixels (0,0) (1,0) and (0,1) This new algorithm does a 2x2 box convolution against the 3x3 pixel neighborhood. It takes 9 tex lookups but produces much nicer results.
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