Use external PNG crate.

[nwin]

Image::png is now just a little layer over an external codec crate.

Closes #390 #300 #400 #414

[nwin]

Note this is WIP. I will move the png crate to PistonDevelopers before merging.

[tomaka]

:(

According to one of the Servo devs (don't remember who exactly), using C code defeats the purpose of using image in the first place.

[nwin]

Servo already depends on flate2. At least @kmcallister seemed to be fine with it.

[bvssvni]

@nwin You are awesome!

[bvssvni]

@tomaka I opened #419

[LaylBongers]

Having a pure rust implementation for most things would be preferable in my opinion.

Edit for more explanation: It wouldn't be an "Imaging library written in Rust" anymore. That it's in Rust and not something else should in my opinion be considered an important feature. For some platforms, dependency management to C isn't that easy. Especially on Windows, C libraries tend to cause nightmares. Next to that, replacing the Rust implementation with a binding to a C one removes the benefit Rust gives with the safety guarantees.

Edit 2: It seems I partly misunderstood what this is doing and what's bringing the C code in play. My bad for not doing proper research before commenting. Still, my problem with it stands that a C dependency would cause more problems than it's worth.

[bvssvni]

@LaylConway See #419

[nwin]

So the updated PNG crate now uses flate2 only for encoding.