Interface restrictions for gif and jpeg

[HeroicKatora]

Hide some public interface dependencies on gif and jpeg. Note that frame
oriented functions are now unused and will likely be brought back in a
(slightly less generic) manner that is more uniform over other multi
frame formats as well.

[HeroicKatora]

I just realized that there's a bug hidden in the interface. Note how several interfaces such as the io in free_functions.rs calls gif::Frame::from_rgb directly and without checking the color type and lossy-casting the width and height with as u16.

image/src/io/free_functions.rs

Line 146 in 737a05d

"gif" => gif::Encoder::new(fout).encode(&gif::Frame::from_rgb(width as u16, height as u16, &buf)),

So, I'll rewrite the interface to align to png::PngEncoder::write_image and make this a bug-fix as well.