Pass through PNG data unchanged

[adetaylor]

I'm told that some tools carefully optimize PNG data (optipng, advpng). At the moment, this library always decodes and re-encodes PNGs before inserting the data into the icns file, which loses those optimizations.

I had a go at this here: master...adetaylor:png-passthrough-unchanged

but it's not product-quality, for several reasons:

• It unwrap()s in bad places, because we're no longer encoding and decoding PNGs in locations where we could return an error. In general therefore the API needs some work.
• I haven't yet attempted to respect the pngio optionality though this should be straightforward.
• The golden.rs tests are now a bit hideous. The PNG data within the tests/icns file did not match the PNG data within the tests/png directory. The tests only passed because they were relying on the old behavior of decoding and re-encoding PNGs so that they subsequently matched. Therefore some gross hackery has been done to force a decode and re-encode.

I'm unlikely to get around to tidying this up enough to upstream, but I thought I'd post this here in case anyone else encounters the same issue and wants to try.