Renamed "heatmap-surface" to "countour" and made big performance improvements.

In particular, the performance gains come from:
- Reusing triangles from previous rendering, instead of delete+recreate every time
- Manually managing SVG gradients
- Hacking the renderer to say isHidden() is false. Not sure why, but it thinks it is hidden (it is not!) and it causes a major performance hit thanks to `chart.cloneRenderTo()`.