Prompt before allowing HTML5 Canvas image extraction

The HTML5 canvas can be used for fingerprinting WebGL and font rendering as described in http://www.w2spconf.com/2012/papers/w2sp12-final4.pdf. The fingerprint technique hinges on the ability for JS to extract image/data urls from the canvas object and hash them and/or compute differences. There's some demonstration code that works for a specific (but currently unknown) ruby version here: https://github.com/kmowery/canvas-fingerprinting.

I think the least-effort defense for now is to simply prompt before image extraction, and to allow extraction permissions to be set on a url-bar domain basis if the user has opted to store browser state to disk.

Later, we can think about virtualizing this surface during extraction, but I don't think we'll need to do that unless every site in the world decides to make a lolcat captioning HTML5 widget.