Large optimizations so updated work is only relative to newly added points, not the total number of points

[danielgjackson]

1. Only newly-added data points need to be drawn to the canvas.
2. Only the "dirty rectangle" around the newly added points has to be re-colorized and drawn on to the canvas
3. Some changes (e.g. radius/blur/resize) will still trigger a full redraw.

[mourner]

This is cool, but I don't feel good about merging. The performance benefits only cover a very rare use case where you're animating a heatmap where points are added one by one (and not removed), but increases code complexity considerably. I'd want to keep this module as small and simple as possible.

For great performance on tons of changing points, you'd still have to employ a more sophisticated technique like gridding (as done in https://github.com/Leaflet/Leaflet.heat).