performance experiments: lazy node drawing; faster edge rendering

these have been in the back of my mind for a while, prompted by b5e17dc40109e1ec877345464c0ebf9a974ed561 comment:

edges seem to render slowly: if I remove their rotation css transform, panning is much faster (but obviously the edges all point the wrong way). Figure out how to optimize this better - maybe remove the transform and the blur-filter that requires it, or figure out how to make the blur work for horiz/vert lines without the transform, or render all the unselected edges as a single svg path instead of 700 different objects.
lazy graph rendering in #36 was a huge performance boost. What if we did that for each node?
What if we didn't try to render objects that are offscreen? I'd expect the browser/svg renderer optimizes for that already, but I have no hard evidence.

erosson / ch2plan