A test release for 8.15.

Changes for this release:

• add support for target_clones attribute [lovell]
  • use with (un)premultiply for ~10% perf gain on AVX CPUs
  • use with XYZ to LAB colourspace conversion for ~10% perf gain on AVX CPUs
• add fast path to extract_band and bandjoin for uchar images [lovell]
• reduce vips_sharpen max sigma to 10 [lovell]
• inline scRGB to XYZ colourspace conversion, ~2x faster [lovell]
• set "interlaced=1" for interlaced GIF images [kleisauke]
• add @line_art to find_trim [miltoncandelero]
• improve C++ binding [MathemanFlo]
  • add inplace() / VImage::new_from_memory_copy()
  • add overloads for draw_*() / VImage::thumbnail_buffer()
• allow negative line spacing in text [donghuikugou]
• add VIPS_META_BITS_PER_SAMPLE metadata, deprecate the
  "palette-bit-depth" and "heif-bitdepth" meta fields [MathemanFlo]
• add "revalidate" to foreign loaders [jcupitt]
• add premultiplied option to smartcrop [lovell]
• add "prewitt" and "scharr" edge detectors, "sobel" is more accurate for
  non-uchar formats [jcupitt]
• add support for forms in pdfium loader [kleisauke]
• swap built-in profiles with ICC v4 variants [kleisauke]
• remove libgsf dependency in favor of libarchive [kleisauke]
• better chunking for small shrinks [jcupitt]
• use alpha range of 0.0 - 1.0 for scRGB images [DarthSim]
• add support for 16-bit float TIFFs [DarthSim]
• add direct mode to dzsave [jcupitt]
• require C++11 as a minimum standard [kleisauke]
• add support for SIMD via Highway [kleisauke]
• threaded write in tiffsave for tiled JPEG and JPEG2000 [jcupitt]
• add vips_thread_execute() to the public API [jcupitt]
• add "keep" flag to foreign savers, deprecate "strip" [a3mar]
• improve scRGB handling [jcupitt]