- update to 8.13.1:
* fix im7 feature detection in meson
* add a summary table at the end of configure in meson
* fix libpng fallback when spng is disabled in meson
* add "unlimited" to jpegload
* better 0 detection in unpremultiply
* fix low bitdepth spng save [jeffska]
* fix PNG low bitdepth save of high bitdepth images
* add support for libjxl 0.7 [kleisauke]
* improve compatibility with older libgsf versions [kleisauke]