Fix `vips_isprefix()` argument order (libvips#4225)

The arguments were reversed, likely due to confusion with
`g_str_has_prefix()`. Ensures all metadata is removed when saving
with `keep=none`.

See: libvips#4224.

Merge branch 'master' into 8.16

branch for 8.16.0-rc1

version bump

fixing the C++ soname issue

version bump

fixing the C++ soname issue

version bump

fixing the C++ soname issue

add release date for 8.15.4

update changelog ready for release 8.15.3

as uploaded

update changelog for release