[matmen]

Adds WebP decoding (lossy, lossless, alpha, animated) and lossless WebP encoding, all in plain TypeScript.

Depends on the Animation PR (it uses Animation for animated WebPs) and is based on its branch. Merge that one first, then retarget this PR to master.

Decoding

• Formats:
  • lossless (VP8L, RFC 9649)
  • lossy (VP8, RFC 6386)
  • alpha (ALPH chunk: raw or VP8L compressed, all filter methods)
  • animations (ANIM/ANMF)
• Still WebPs: Image.decode(), including the Exif orientation like the other formats.
• Animated WebPs: Animation.decode() composites the frames the way libwebp's animation decoder does. Image.decode() refuses them with a pointer to Animation.decode(), and decode() returns them as Animation.
• Detection: ImageType.isWebP(), and ImageType.getType() returns 'webp'.
• Speed (12 MP photo): lossy decodes in ~0.8 s, lossless in ~0.8 s.

Encoding (lossless)

• Image.encodeWebP(): files are about 25% smaller than our PNGs at compression level 6.
  • up to 256 colors: palette, with several pixels packed per byte for up to 16 colors
  • otherwise: subtract green and a predictor picked per 16×16 block
  • entropy coding: LZ77 (including the short 2D distance codes), color cache, length-limited Huffman codes
• Animation.encodeWebP(): writes animated WebPs.
  • WebP has no "previous" disposal and stores frame offsets halved. Animations that rely on either are written as full composited frames, so they always look as intended.
• Not included: lossy encoding.

Verification

• libwebp test vectors: all 96 files of the official libwebp-test-data that have reference checksums decode bit-exactly to dwebp's output. That covers lossy, lossless, alpha, segmentation, both loop filters and all transforms.
• Pillow (libwebp 1.6) on the WebPs from the Pillow, image-rs and libwebp test suites: 97 of 98 files are pixel-identical, animations included. The remaining file (bad_palette_index.webp) matches libwebp's reference checksum, Pillow handles the invalid index differently.
• Encoder: 172 images (PngSuite, photos, screenshots, logos) were encoded and then decoded by libwebp. All of them are lossless and identical to our own decoder's output.
• Robustness: 3000 mutated files (bit flips, truncation, random bytes) only raise errors: no crashes, no hangs, slowest 158 ms.
• New tests: tests/webp.js, with fixtures written by libwebp and checksums of libwebp's decoded pixels.

Credits

The VP8 constant tables are extracted from RFC 6386. The YUV→RGB conversion, the "fancy" chroma upsampling and the blending of animation frames reproduce libwebp's integer arithmetic, so the output matches libwebp exactly. Both are BSD-licensed and credited in THIRD_PARTY_NOTICES.md and the source headers.
The WebP container and the lossless codec follow RFC 9649, whose short distance code table is included under the Revised BSD License.

Package size grows from ~112 kB to ~140 kB (compressed).

Closes #19
Closes #20