Some filter/renderPass disable GL_BLEND but never restore them
Namely :
- vtkDepthOfFieldPass.cxx
- vtkEDLShading.cxx
- vtkPointFillPass.cxx
- vtkSobelGradientMagnitudePass.cxx
- vtkSSAAPass.cxx (!3003 (merged))
Hence if used with something like vtkTextActor or vtkBalloonRepresentation the blending is disabled and the rendering incorrect