Fix against build error with OpenGL ES 2.0
Fix against build error with OpenGL ES 2.0

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