Animation class + APNG support - #48

Adds Animation, a format-independent base class for animated images, and APNG decoding/encoding. GIF becomes a subclass of Animation and is deprecated in its favor.

New

• Animation: an array of Frames with loopCount, width/height/duration, resize() and slice(). This is the same API GIF had, but not tied to one format.
  • Animation.decode(data) decodes GIFs and APNGs into fully composited frames. Still images become a single frame.
  • encodeGIF(quality) and encodeAPNG(compression) encode it.
• APNG decoding: all frames with their delays, offsets, dispose and blend operations, plus the loop count.
  • Broken animations (wrong sequence numbers, missing frame data, wrong frame count) fall back to the default image, as browsers do.
  • Image.decode() keeps returning an APNG's default image.
• APNG encoding: lossless, with full alpha.
  • All frames share the smallest color type that fits them: a palette for up to 256 colors, otherwise greyscale or RGB, with or without alpha.
  • The first frame doubles as the default image for viewers without APNG support.

Deprecated

• GIF: use Animation instead. new GIF(frames).encode() becomes new Animation(frames).encodeGIF(), and GIF.decode() becomes Animation.decode().
  • GIF keeps working unchanged in 3.x, and decode() still returns GIF instances for GIF files.
  • Removal is planned for 4.0.

Testing

• Pillow's APNG test suite (54 files):
  • 38 are pixel-identical to Pillow.
  • 4 differ only where Pillow blends semi-transparent pixels differently from the APNG spec's integer "over" formula, which we follow.
  • 11 are broken files Pillow rejects; we fall back to the default image, like browsers.
• Round trips: animations with offsets and all disposal modes (keep, background, previous), encoded as APNG and decoded again by us and by Pillow, match the expected composited frames exactly.
• Loop counts: -1/0/n round-trip through APNG and GIF.
• New tests: tests/animation.js.

Closes #6