vtkOpenGLRenderWindow: bind default frame buffer

vtkOpenGLRenderWindow never bound the default frame buffer when
rendering began. While this is generally not necessary, if for some
reason the bound frame buffer is changed then vtkOpenGLRenderWindow ends
up rendering to incorrect frame buffer. Fixed that.

Rendering/OpenGL2/vtkOpenGLRenderWindow.cxx

@@ -1276,8 +1276,7 @@ void vtkOpenGLRenderWindow::Start()
     this->BackRightBuffer  = buffer;
     this->FrontRightBuffer = buffer;
   }
-
-  if (!this->UseOffScreenBuffers && this->OffScreenFramebufferBound)
+  else if (!this->UseOffScreenBuffers && this->OffScreenFramebufferBound)
   {
     this->OffScreenFramebuffer->RestorePreviousBindingsAndBuffers();
     this->OffScreenFramebufferBound = false;
@@ -1288,6 +1287,11 @@ void vtkOpenGLRenderWindow::Start()
     this->BackBuffer = static_cast<unsigned int>(GL_BACK);
     this->FrontBuffer = static_cast<unsigned int>(GL_FRONT);
   }
+  else
+  {
+    // makes sense to activate the default framebuffer.
+    glBindFramebuffer(GL_FRAMEBUFFER, this->GetDefaultFrameBufferId());
+  }
 }