vtkGenericOpenGLRenderWindow: add PushState/PopState methods
vtkGenericOpenGLRenderWindow: add PushState/PopState methods

These allow users of the class to not care about OpenGL versus the
OpenGL2 backend.

SimVascular/VTK,sumedhasingla/VTK,SimVascular/VTK,gram526/VTK,SimVascular/VTK,sankhesh/VTK,keithroe/vtkoptix,sumedhasingla/VTK,msmolens/VTK,jmerkow/VTK,SimVascular/VTK,sankhesh/VTK,demarle/VTK,SimVascular/VTK,demarle/VTK,jmerkow/VTK,keithroe/vtkoptix,demarle/VTK,jmerkow/VTK,sumedhasingla/VTK,sankhesh/VTK,sankhesh/VTK,jmerkow/VTK,keithroe/vtkoptix,sumedhasingla/VTK,msmolens/VTK,gram526/VTK,demarle/VTK,sankhesh/VTK,mspark93/VTK,msmolens/VTK,keithroe/vtkoptix,mspark93/VTK,mspark93/VTK,gram526/VTK,keithroe/vtkoptix,msmolens/VTK,SimVascular/VTK,keithroe/vtkoptix,jmerkow/VTK,msmolens/VTK,msmolens/VTK,demarle/VTK,mspark93/VTK,keithroe/vtkoptix,SimVascular/VTK,msmolens/VTK,sankhesh/VTK,demarle/VTK,jmerkow/VTK,mspark93/VTK,jmerkow/VTK,gram526/VTK,gram526/VTK,jmerkow/VTK,sankhesh/VTK,sumedhasingla/VTK,msmolens/VTK,mspark93/VTK,sumedhasingla/VTK,mspark93/VTK,sumedhasingla/VTK,gram526/VTK,gram526/VTK,demarle/VTK,sumedhasingla/VTK,gram526/VTK,keithroe/vtkoptix,sankhesh/VTK,demarle/VTK,SimVascular/VTK,mspark93/VTK