Handle OBJ-file materials without textures.
When a material has no texture filename, no vtkTexture instance should be added to the actor.
This patch is based off v6.3.0 and should be merge-able to both master and the 6.3 branch.
When a material has no texture filename, no vtkTexture instance should be added to the actor.
This patch is based off v6.3.0 and should be merge-able to both master and the 6.3 branch.