Progress towards sharp v0.34.0

[lovell]

See the hat branch for work-in-progress.

Done so far:

• Upgrade to libvips v8.16.0
• Expose isPalette metadata - see Add VIPS_META_INDEXED meta libvips/libvips#3756
• Expose WebP smartDeblock output option - see WebP Save: add argument for using "auto-filter"/"smart_deblock" encoding feature. libvips/libvips#3852
• Expose recent libjxl features such as animation, EXIF metadata (requires global libvips compiled with support for libjxl)
• Add linux-ppc64le to supported platforms
• Upgrade to C++17 - see Windows: support info.size on wide-character systems (a.k.a. the C++17 upgrade) #3943
• Change format of metadata.background for greyscale images - see Enhancement: ensure metadata.background is compatible with color package for single-channel images #4090
• Upgrade to libheif v1.19.1 for AOM auto-tiles feature - see aom encoder: expose auto-tiles boolean option (since v3.10.0) strukturag/libheif#1330

Open pull requests waiting for review:

• Merge autoOrient improvements - see Rotation sugar and more #4151
• Merge erode/dilate feature - see Erode / dilate preliminary implementation #4243

Possibly maybes (PRs welcome):

• Expose libvips' new Signed Distance Fields (SDF) feature? Can be used for shaped masks, removing need for SVG, will require some documentation.
• Increase AVIF default quality, would benefit from some quantitative research - see How were the default AVIF quality and effort settings chosen? #4227