message: Fix GIF transparency
diff: @@ -489,7 +489,7 @@ impl<W: Write> GifEncoder<W> {
  )))
  }
  - pub(crate) fn encode_gif(&mut self, frame: Frame) -> ImageResult<()> {
  + pub(crate) fn encode_gif(&mut self, mut frame: Frame) -> ImageResult<()> {
  let gif_encoder;
  if let Some(ref mut encoder) = self.gif_encoder {
  gif_encoder = encoder;
  @@ -504,6 +504,8 @@ impl<W: Write> GifEncoder<W> {
  gif_encoder = self.gif_encoder.as_mut().unwrap()
  }
  + frame.dispose = gif::DisposalMethod::Background;
  +
  gif_encoder.write_frame(&frame).map_err(ImageError::from_encoding)
  }
  }
  