Alpha heuristic improvements #144
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I've found an image that with alpha reductions is compressed worse than without alpha reductions (I can't share the image, as it's private). That's because zlib strategy = 2 ignores repetitions, and picks alpha that compresses badly with a better strategy.
Cloning of image makes copy of both idat and raw data, but in alpha reductions both are getting replaced immediately, which is wasteful. I've made alpha reductions return only copy of relevant data.