Add the update.sh for third-party libpng.
Add the update.sh for third-party libpng.

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