Fix issue exporting cell-data colors for composite datasets.
Fix issue exporting cell-data colors for composite datasets.

When vtkX3DExporter encountered composite datasets, it failed to
correctly determine if cell-data colors were being used. As a result the
exported file had invalid colors when cell-data coloring was enabled.
This fixes that issue.

keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK