revamp obj reader for ospray
Many obj files contain multiple objects with different materials. VTK's reader produces a single concatenated object and ignores the materials with the exception of texture coordinates. We should revamp the reader to produce multiblock of polydata so that applications can work with each object independently. At the same time we should create a tga reader for texture files and lastly parse the mtl files and populate a vtkospray material definition library from them.