[VTK] Restore offscreen parameters and test vtkmodules

[xavArtley]

fix #1401

[codecov]

Codecov Report

Merging #1402 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1402   +/-   ##
=======================================
  Coverage   87.50%   87.50%           
=======================================
  Files         131      131           
  Lines       14072    14074    +2     
=======================================
+ Hits        12313    12315    +2     
  Misses       1759     1759           

Impacted Files	Coverage Δ
panel/pane/vtk/vtk.py	87.69% <100.00%> (ø)
panel/pane/vtk/vtkjs_serializer.py	82.16% <100.00%> (+0.11%)	⬆️

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[philippjfr]

Sorry, thought I'd merge your major VTK refactor first and obviously now there are conflicts.

[xavArtley]

Not a problem
It's some small changes that I will report

[xavArtley]

superseded by #1406