Use pre-rendered arcs (big performance increase)

I've tweaked the heatmap.js code to render the circle+shadow once (on a third canvas) then use a combination of drawImage() and globalAlpha to replace the repeated circle draws. The performance increases are significant- check out the before:

http://alastaircoote.github.com/heatmap.js/tests/performance-original.html

and after:

http://alastaircoote.github.com/heatmap.js/tests/performance-copyandpaste.html

More details in a blog post here:

http://blogging.alastair.is/improving-performance-never-underestimate-copy-and-paste/