Add setting attribute to disable reading from canvas

Canvas fingerprinting is a way (one of many) to gather entropy for fingerprinting users based on how their browser acts. Most measures against fingerprinting can be modified at runtime (e.g. by modifying the user-agent header), but canvas fingerprinting can only be avoided by passing the --disable-reading-from-canvas flag to the underlying Chromium (try e.g. visiting https://browserleaks.com/canvas with/without that flag with simplebrowser).

It would be great if there was a QWebEngineSettings::WebAttribute to set it at runtime, so that it can be disabled when loading a page where it causes issues (e.g. playback controls disappearing on Youtube).