[Paraview] Differences between OpenGL and OpenGL2 versions of ParaView

[Utkarsh Ayachit]

> I did notice PV is forcing the actor to be modified every time the LOD
> changes (which is a lot). Modifying the actor causes the VBO/IBOs to rebuild
> so it would be better not to do that unless you really need to and in this
> case I'm not sure why it is being done. Specifically I think it is in
> vtkPVLODActor::Modified where it calls this->Device->Modified() . The LOD
> actor is being modified as a result of calling SetEnableLOD constantly
> flipping between true and false. That is not the cause of this issue but it
> is something I noticed that will be a pain for large datasets.

Ken, currently, there's no way around it. vtkPVLODActor works by
changing the mapper on the internal vtkActor aka Device when LOD mode
is toggled. That will indeed change the Device's MTime even if we stop
calling this->Device->Modiied() in the vtkPVLODActor::Modified(). The
question is why are VBO/IBOs rebuild if actor MTime changes? VBOs
would be data dependent right? Most of data-dependent properties are
on  the Mapper, not the actor. Shouldn't the code that re-creates
VBO/IBOs only depend on Mapper's MTime?

Utkarsh