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.
It was happening automatically, not by any manual call to `update`. Specifically, when we have a single visual shown in two ViewBoxes, it is assigned a new transform each time it is drawn to a different viewbox. This assignment causes the visual to request an update, which causes the canvas to redraw and repeat the process again. By blocking that update signal during the draw, the loop is broken.