v8.15.0-rc1

Release 8.15.0-rc1

• Update dependencies to latest where possible.
• Select optimisation level per dependency (#56).
• Add new '-static-ffi' variant (#57).
• Add libdicom to 'all' group dependencies.
• Add highway to 'web' group dependencies.
• Remove liborc in favor of highway.

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