vtkValuePass: save and restores GL state to fix rendering issues

When initializing the FBOs, blending, scissor testing, and depth
testing are disabled for the first render, which leads to junk results
on the first render. Fix by saving and restoring that OpenGL state.

Rendering/OpenGL2/vtkValuePass.cxx

@@ -621,6 +621,13 @@ bool vtkValuePass::InitializeFBO(vtkRenderer* ren)
   this->ImplFloat->ValueFBO = vtkOpenGLFramebufferObject::New();
   this->ImplFloat->ValueFBO->SetContext(renWin);
   renWin->GetState()->PushFramebufferBindings();
+
+  // vtkOpenGLFramebufferObject::InitializeViewport disables blending, scissor test and
+  // depth test. Save and restore them in scoped variables.
+  vtkOpenGLState* glstate = renWin->GetState();
+  vtkOpenGLState::ScopedglEnableDisable blendState(glstate, GL_BLEND);
+  vtkOpenGLState::ScopedglEnableDisable depthTestState(glstate, GL_DEPTH_TEST);
+  vtkOpenGLState::ScopedglEnableDisable scissorTestState(glstate, GL_SCISSOR_TEST);
   this->ImplFloat->ValueFBO->Bind();
   this->ImplFloat->ValueFBO->InitializeViewport(size[0], size[1]);
   /* GL_COLOR_ATTACHMENT0 */
@@ -635,7 +642,7 @@ bool vtkValuePass::InitializeFBO(vtkRenderer* ren)
     return false;
   }
 
-  renWin->GetState()->PopFramebufferBindings();
+  glstate->PopFramebufferBindings();
   this->ImplFloat->FBOAllocated = true;
 
   return true;