Firefox 58 will default to require permissions for reading canvas data

[JobLeonard]

https://www.ghacks.net/2017/10/28/firefox-58-warns-you-if-sites-use-canvas-image-data/

https://bugzilla.mozilla.org/show_bug.cgi?id=967895

This will likely break a lot of stuff. For example, it has already popped up in this issue in RES (which is how I found out about this library).

Not sure if there is any workaround other than the user giving this permission. It might be worth adding to the documentation at least, so people don't have to hunt as long for reasons why the library isn't working properly.