Tor Browser canvas fingerprint pop up should have "not ever" option and timeout

Hi.

Currently, when accessing a site Tor Browser thinks is doing canvas fingerprinting, it pops up a little thing asking if you'd like to allow it:

1. not now (default)
2. never for this site
3. allow in the future

This is excellent, and I'm super excited we're given such granular choices.

I'd like an additional option, which is: 4. Block all canvas fingerprinting, on any site, ever.

There exists no scenario where I wish to be fingerprinted through canvas feature usage. I've spoken to others who feel the same.

It makes sense to me to place it either in the about:config or the security slider (perhaps enabled at any setting higher than "low").

Trac:
Username: Aranjedeath