The codec library doesn't use any external dependencies. The PNG crate
is only to load some source material for testing the codec. Turning the
test driver into a real test lets me turn the PNG dependency into a
dev-dependency.
I've pulled in the PNG crate to write the images back out as regular
PNG images. Now I can compare the results of my decoder against the
reference images.
I also made a weird utility iterator thing to wrap the Decoder's pixel
iterator because the PNG crate wants a &[u8], not a bunch of u32's or
custom pixels structs. I should learn how to impl the right traits so
that I can just `.to_components()` on an Iterator<Item=PixelRGBA>
instead of the wrapper constructor thing I have now.
