Optimize vtk

makes things a bit faster, still can be improved in another branch for template rendering and colorbar rendering.

@dlonie it is very similar to what you looked at before, should be able to review and merge this easily.

@aashish24 please take a look as well.

Don't forget to approve sister pull request: https://github.com/UV-CDAT/uvcdat-testdata/pull/8

I don't mind moving the finalize call somewhere else looking at VTK doc I couldn't figure out what it exactly does, the doc says:
```virtual void vtkRenderWindow::Finalize	(		)	
pure virtual
Finalize the rendering process.

Implemented in vtkCocoaRenderWindow, vtkCocoaRenderWindow, vtkIOSRenderWindow, vtkCarbonRenderWindow, vtkXOpenGLRenderWindow, vtkOSOpenGLRenderWindow, vtkOSOpenGLRenderWindow, vtkXOpenGLRenderWindow, vtkWin32OpenGLRenderWindow, vtkWin32OpenGLRenderWindow, vtkEGLRenderWindow, and vtkGenericOpenGLRenderWindow.
```
see:

http://www.vtk.org/doc/nightly/html/classvtkRenderWindow.html#a8c7d3378689bac954bca95051273bc74