Fix scenegraph examples

Minor changes to reduce CPU usage in scenegraph examples. 
Also made `viewbox.py` resize with the canvas, and cleaned up `nested_viewbox.py`
Ready for review.
Generally the user never needs to call any `update` methods when using the scenegraph. These should be invoked automatically whenever a redraw is needed. We call `Canvas.update()` only in the gloo and visual examples.