message: Gif example updates
diff: let logical_screen_descriptor = {
  image_width <- u16le,
  image_height <- u16le,
  - flags <- u8, // TODO: interpret flags
  + flags <- u8,
  bg_color_index <- u8,
  pixel_aspect_ratio <- u8,
  +
  + // TODO: interpret flags
  + // has_color_table : Bool = (flags & 0b10000000) != 0,
  + // color_table_size : U16 = 2 << (flags & 7),
  };
  /// # Header
  @@ -46,7 +50,6 @@ let color_table_entry = {
  ///
  /// - [GIF89a Specification: Section 19](https://www.w3.org/Graphics/GIF/spec-gif89a.txt)
  let global_color_table = fun (len : U16) => {
  - // TODO: use a function rather than a struct
  entries <- array16 len color_table_entry,
  };
  