message: fix bug of skip_palette
diff: @@ -1087,7 +1087,7 @@ impl<R: Read + Seek> BmpDecoder<R> {
  |row| {
  reader.read_exact(&mut indices)?;
  if skip_palette {
  - row.clone_from_slice(&indices);
  + row.clone_from_slice(&indices[0..width]);
  } else {
  let mut pixel_iter = row.chunks_mut(num_channels);
  match bit_count {
  @@ -1462,7 +1462,9 @@ impl<'a, R: 'a + Read + Seek> ImageDecoder<'a> for BmpDecoder<R> {
  }
  fn color_type(&self) -> ColorType {
  - if self.add_alpha_channel {
  + if self.skip_palette {
  + ColorType::L8
  + } else if self.add_alpha_channel {
  ColorType::Rgba8
  } else {
  ColorType::Rgb8
  