Adapt vtkActor to correctly detect translucent geometry on mapper

- address paraview/paraview#22605
- fixes a bug that happened when using the composite polydata mapper
  with per-block overriden opacities. When the actor was made transparent,
  and a block's opacity was overriden to 1.0, that block was never
  rendered, because the renderer inferred from the actor that there was
  no need to render opaque geometry.
- this commit also adds a unit test that cycles opacity for individual
  blocks of a translucent actor and verifies the rendered image is
  correct.