GIF decoder: LZW, frames, transparency #69

Implement a pure Rust GIF decoder in the image crate per GIF89a specification.

Requirements#

• Parse GIF header and logical screen descriptor
• Global and local color tables
• LZW decompression (variable-width codes, clear code, end-of-information code)
• Image descriptor and frame data
• Transparency via Graphic Control Extension (transparent color index)
• Multi-frame support: decode all frames with disposal methods and delay times
• Frame struct: image data, delay, disposal method
• Output first frame as RGBA8 Image, expose frame iterator for animations
• API:
  • fn decode_gif(data: &[u8]) -> Result<Image, ImageError> (first frame)
  • fn decode_gif_frames(data: &[u8]) -> Result<Vec<GifFrame>, ImageError> (all frames)

Acceptance Criteria#

• Decodes single-frame and animated GIFs
• Handles transparency correctly
• LZW decompressor handles code table resets and variable-width codes
• Unit tests for LZW decompression, color table parsing, frame decoding
• Integration tests with real GIF files
• No external dependencies, no unsafe

Dependencies#

• Requires pixel format types

Phase 7 — Image Decoders