png decoder

[Lutger]

Perhaps somebody finds it useful: I've ported LodePNG, a small and 
simple png decoder to D.(1 function call, life doesn't get easier than 
this). There's also an encoder part, which I have not ported yet.

You can get it here: lutger.ifastnet.com/png.d
Documentation: lutger.ifastnet.com/png.html

Port is straightforward, no attempt to change things much except some 
raw pointers to arrays and replaced zlib routine by Phobos ones. Also 
not tested much.