OpenGL error hunt -- dashboard cleanup

Use static version of vtkOpenGLCheckErrorMacro in helper class
that's not a vtkObject in vtkPistonMapper. Don't check for OpenGL
errors in vtkGL2PSExporter::Write it doesn't use OpenGL directly.
Fix warning about narrowing cast. Use vtkOpenGLCheckErrorMacro
before return in vtkOpenGLImageSliceMapper::TextureSizeOK.

Change-Id: Ia05575d571c230b68b2035e8a866f0ec7519d887
Merge topic 'opengl-error-hunt' into master

6455c581 OpenGL error hunt -- dashboard cleanup