vtkUnstructuredGrid (.vtu data) with vtkJS

[thewtex (Matt Mc Cormick)]

@Sebastien_Jourdain

Such filter could be brought onto the client by building a WebAssembly module from VTK/C++.

This is available in itk.js:

https://insightsoftwareconsortium.github.io/itk-js/docs/polydata_formats.html