It was returning either pixel size or compressed block size, which is
now available directly via other means.
This is a breaking change, but I don't expect these functions to be
used widely beyond Magnum internals.
Too much burden to implement. Nope. Sorry. All APIs were just asserting
that it's not enabled at the moment, so I may as well just remove it
completely.
Currently just does per-pixel comparison and calculates absolute delta,
failing the comparison if max/mean delta threshold is above specified
values. Useful enough for the case I have right now, might fail in other
case -- but still better than whatever else I was using before :)