v8.18.0-rc2

Release 8.18.0-rc2

• Update dependencies to latest where possible.
• Add libultrahdr to 'web' group dependencies for Ultra HDR support (#88).
• Add LibRaw to 'all' group dependencies for RAW digicam input support (#77).
• Add zstd to 'all' group dependencies for Zstd-compressed TIFF support (#80).
• Drop spng from dependencies (#84).
• Distribute missing vips-dev-arm64-all-*.zip and vips-dev-arm64-web-*.zip packages.

PDBs can be found here:
https://libvips-packaging.s3.amazonaws.com/pdb/index.html