Please dispose of GIF frames properly

This crate implements GIF disposal method for the gif crate.

The gif crate only exposes raw frame data that is not sufficient to render GIFs properly. GIF requires special composing of frames which is non-trivial.

```rust let file = File::open("example.gif")?; let mut decoder = Decoder::new(file);

// Important: decoder.set(gif::ColorOutput::Indexed);

let mut reader = decoder.read_info()?;

let mut screen = Screen::new(&reader); while let Some(frame) = reader.readnextframe()? { screen.blit(&frame)?; screen.pixels // that's the frame now } ```