Changed vtkRenderer::Render() to vtkRenderWindow::Render()

Previous code caused PCLVisualizer to crash on OS X. According to VTK
documentation it is forbidden to call Render() on vtk::Renderer.
Correcting variable name.
Fixed variable names
Merge pull request #384 from jakobwilm/visualizer-fixes

Changed vtkRenderer::Render() to vtkRenderWindow::Render()