Support GIF output and palette PNG output

[zoellner]

Fixes #13.

Problem

Saving a GIF fails with VipsOperation: class "gifsave_buffer" not found, and png({ palette: true }) quietly writes a truecolour PNG. Stock sharp (prebuilt binaries) supports both, so switching to this layer silently drops them.

libvips 8.18 writes GIF files through cgif, and its meson.build only looks for cgif once an image quantiser (libimagequant or quantizr) has been found. Palette PNG output needs the same quantiser; without one libvips logs ignoring palette (no quantisation support) and carries on. The layer built neither.

Changes

• layer/Makefile
  • Build libimagequant 2.4.1 (lovell/libimagequant, the BSD-2-Clause fork used by sharp's prebuilt binaries; upstream 4.x is GPL-3.0) and cgif 0.5.4 (MIT), both from pinned tarballs with SHA256 checks, the same versions sharp-libvips uses.
  • Pass -Dimagequant=enabled -Dcgif=enabled to libvips. Without forcing the quantiser on, a missing libimagequant would quietly disable cgif instead of failing the build.
  • verifyLibvips now also checks for gifsave_buffer.
  • Both libraries are built as shared libraries into /usr/local/lib, and the existing ldd-based step copies them into the layer's lib/.
• test/layer.test.js
  • The format support test now also covers gif.
  • Resize the synthetic HEIC to GIF and check the size, the GIF89a signature and the quadrant colours.
  • Build a 2-frame animated GIF, then run the pipeline from Can't save GIFs - gifsave_buffer not found #13 on it (sharp(buf, { animated: true }).resize().gif()). Check the frame count and size, and the colours of both frames.
  • Encode a palette PNG and check that the IHDR colour type is 3 (indexed) and isPalette is true. Because libvips silently falls back to truecolour, checking only that the call succeeds would not catch a missing quantiser.
• README.md: a note on GIF and palette PNG output, plus license and repository entries for cgif and libimagequant.

Testing

• The new tests pass locally against stock prebuilt sharp 0.35.5, run on a PNG version of the quadrants fixture because stock sharp can't decode HEVC.
• With palette: true removed, the palette PNG test fails as expected (PNG colour type should be indexed).
• The full layer build and the tests in the Lambda runtime image run in the Test layer workflow on this PR.