Add PNG write-out machinery

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.

Cargo.toml

@@ -6,3 +6,4 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+png = "0.17.14"