fix grid-absolute for off-center models

[snoyer]

absolute-grid option currently works as intended when models are off-center vertically (in the up direction) but not when they are off-center "sideways" (along the grid plane), see example below:

This PR changes the behavior to the following:

axes are still in the same position at the origin, but the actor and fading radius of the grid are centered below the model.

There is still room for improvement, for example if the model is very far from the origin the axes may not be visible at all (outside of radius), solving that would require more involved framing/fading patterns of the grid plane.

This fix should at least make the feature not broken anymore.

[codecov]

Codecov Report

Merging #1049 (4fe3f9b) into master (07d6c6e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1049   +/-   ##
=======================================
  Coverage   96.22%   96.22%           
=======================================
  Files         120      120           
  Lines        7015     7023    +8     
=======================================
+ Hits         6750     6758    +8     
  Misses        265      265           

Files	Coverage Δ
...VTKExtensions/Rendering/vtkF3DOpenGLGridMapper.cxx	99.00% <100.00%> (+0.03%)	⬆️
...y/VTKExtensions/Rendering/vtkF3DOpenGLGridMapper.h	100.00% <100.00%> (ø)
library/VTKExtensions/Rendering/vtkF3DRenderer.cxx	99.86% <100.00%> (+<0.01%)	⬆️

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[Meakk]

Indeed, fading based on the model bbox looks better than fading based on the grid origin.