Add compression to PNG

[nwin]

The ability to compress PNG images was reduced in order to make it work with Rust 1.0 beta that should be added back at some point…

[tomaka]

Shouldn't the zlib/deflate encoders/decoders be extracted into their own library?

[nwin]

That would be the optimal solution. I just put this up to document it somewhere else than in some PR comments.

[nwin]

I wonder if we shouldn’t just use flate2 for compression and decompression…

[tomaka]

😢 😭

[nwin]

I would be very happy keep the pure Rust solution. But writing a zlib en-/decoder is hard work. I tried both ours, the one from #99 and the one from alexcrichton/rust-compress. They all have bugs. If we are lucky they are easy to fix. I didn’t succeed.

[tomaka]

I don't think it's that hard to write.

[kvark]

@nwin fixing rust-compress would be nice, if it's failing. We need to add your case as a test.

[nwin]

@tomaka not hard but tedious
@kvark I will try to extract some stream, I can’t remember which images failed it. My guess was that something is fishy with the static Huffman tables. The current testcases of rust-compress are all using dynamic Huffman tables. I tried to make a testcase with zlib but that passed…

[nwin]

Closing in favor of #419.