I'm all for anti-fingerprinting, but i'm also for interactive graphics on the web, and getImageData() is essentially your way of accessing a pixel buffer... I would be better if it was more conditional.
e.g instead gate the call that ultimately attempts to send any derivatives of that data over the network - although I understand that may entail significant complexity in the JS engine. Alternatively, gate getImageData() only if fingerprintable context methods have previously been called, i.e those with antialiasing, compositing, blending differences etc or any other rendering method with potential differences emerging from the underlying algorithm. That way someone just trying to use the pixel buffer as an output doesn't get punished by needlessly causing modals to be thrown in front of the user.