A test version of 8.17 to help nip4 development.

Changes since 8.16:

• add keep_duplicate_frames option to GIF save [dloebl]
• add Magic Kernel support [akimon658]
• tiff: add threadsafe warning/error handlers (requires libtiff 4.5.0+) [lovell]
• tiffload: add support for fail_on flag [lovell]
• tiffload: add support for unlimited flag (requires libtiff 4.7.0+) [lovell]
• much more reliable operation caching
• colour: add support for auto-selecting the rendering intent [kleisauke]
• add matrixmultiply
• improve performance of vips_shrink() [kleisauke]
• svgload: add support for custom CSS via stylesheet option [lovell]
• heifload: unlimited flag removes all limits (requires libheif 1.19.0+) [lovell]
• heifsave: improve alpha channel detection [lovell]
• convi: ensure double sum precision for floats [lovell]