Problem with polydatas sharing vtkPoints in vtk8

Hi,

I'm trying to update from vtk 7.0 and I'm having an issue with a particular
setup of polydatas. In my case I have a single vtkPoints that is shared by
multiple vtkPolydatas, where each polydata renders a different set of
cells. I can't get the rendering to show more than one polydata at a time
in vtk 8.0 and up, but it works as expected in vtk 7.1. In all cases I am
using the new OpenGL2 backend.

I created a minimal example that reproduces the issue for me:
https://gist.github.com/tigarmo/c90ff07a1b63bbcf396fa4aaf1e824a9 . It tries
to show 2 quads, where each quad is a cell in a different polydata, but
both polydatas share the same vtkPoints. This shows 2 quads as expected (by
me) in vtk 7.1 but only 1 quad in vtk 8.0.1, 8.1.0 and 8.1.1. A surprising
detail is that which quad is shown varies depending on the run. Sometimes
it's the left one, sometimes the right one.

Does anyone have any suggestions? Is this a bug? Could it be related to
improvements in VBO caching that I saw in 8.0's changelog?

Thank you for your time,

Tiago Nobrega

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