Fix and update code

[bluddy]

• Update to work with decompress 1.0.0
• Tested by reading and writing the same PNG file.
• Simplified: since we already have an input/output abstraction, there's no need to functorize the PNG writer. Make it symmetric with reader.
• Fix bug in writer: close wasn't being called on the channel, leading to 0 size output files.

[bluddy]

Can I have some feedback on this PR?

[cfcs]

I believe these are already covered by my gigantic unmerged GIF PR (perhaps modulo the write) which also changes various abstractions in order to support animated GIFs / progressive rendering, and that is why I don't feel like merging this since it'd be a painful rebase.

I don't know how @rlepigre feels, and to be clear I'm not opposing this PR, just don't feel motivated to deal with it myself. :(

[rlepigre]

I had a quick look and this looks good to me. I'd be happy to merge this after having a closer look, but I don't want to make @cfcs's like difficult with #24. Moreover, I really don't have much time to work on this project currently, and so I really don't want to make any promise.

[cfcs]

@rlepigre & @bluddy: How would you two feel about if I:

• cherry-pick the relevant changes from @bluddy's present PR into the GIF PR
• since the GIF PR is still not quite production quality, make sure the ImageLib_unix module still calls out to imagemagick (edit: for GIF) to try to keep existing users happy
• update + merge the GIF PR
• then cut an opam release

[rlepigre]

Sounds good to me! Thanks!

[cfcs]

I haven't forgotten about this, having trouble upgrading to decompress.1.1.0, I will speak to @dinosaure about it.

[yomimono]

As a big fan of the gif_prelim branch, let me know if I can help out.