A test release for nip4 development.

Changes since 8.17.0:

• add dcrawload, dcrawload_source, dcrawload_buffer: load raw camera files using libraw [lxsameer]
• add magickload_source: load from a source with imagemagick
• add vips__worker_exit(): enables fast threadpool shutdown
• larger mmap windows on 64-bit machines improve random access mode for many file formats
• pdfload: control region to be rendered via page_box [lovell]