GIF encoding with partial frames and transparency

[Pinta365]

Problem

The GIF encoder did not properly support animated GIFs with partial frames (frames smaller than the
canvas that are positioned at an offset). This caused animated GIF round-trips to produce corrupted
output.

Issues:

1. Frame position (left, top) was always set to (0, 0)
2. Frame dimensions always used canvas size instead of actual frame size
3. Alpha channel was ignored, breaking transparency in partial frames
4. Disposal method from input was not preserved

Solution

src/utils/gif_encoder.ts

1. Added GIFFrameData interface with frame metadata (left, top, width, height, disposal)

2. Updated addFrame() signature to accept optional frame metadata:

   addFrame(data, delay, { left, top, width, height, disposal });

3. Updated quantize() to handle transparency:

   • Detects transparent pixels (alpha < 128)
   • Reserves palette index 0 for transparency
   • Uses actual frame dimensions instead of canvas dimensions

4. Updated encode() to use frame metadata:

   • Writes correct frame position and dimensions in Image Descriptor
   • Sets transparency flag in Graphic Control Extension when needed
   • Preserves disposal method from frame metadata

src/formats/gif.ts

1. Updated encodeFrames() to pass frame metadata to encoder
2. Added mapDisposalMethodToNumber() helper function

Backward Compatibility

The changes are backward compatible. The addFrame() method still works with just (data, delay) -
frame metadata is optional with sensible defaults.