vtkOpenGLExtensionManager *extensions = renwin->GetExtensionManager();
if (extensions->ExtensionSupported("GL_EXT_blend_minmax")) {
extensions->LoadExtension("GL_EXT_blend_minmax");
vtkOpenGLExtensionManager *extensions = renwin->GetExtensionManager();
if (extensions->ExtensionSupported("GL_EXT_blend_minmax")) {
extensions->LoadExtension("GL_EXT_blend_minmax");