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