Update ChangeLog for 8.18 (#4810)

set a release date for 8.18.0

assuming rc3 is the final one heh

fix tiffsave minimise

#4791 set the minimise on the
wrong image, thanks @kleisauke

cplusplus: update Ultra HDR sample after 9e7e75c (#4778)

note version dates

tiffload: ensure docs match memory limit behaviour (#4665)

See commit dcb1df8

enable caching of vips_system() (#4640)

* enable caching of vips_system()

We were not caching vips_system() calls, since they could potentially
have side effects. However, almost all the time we are doing things
like running imagemagick commands, so caching is very helpful.

This patch enables caching of vips_system(), and adds better hashing of
compound argument types.

* vips_linear() was changing its args

the new arg hasher spotted it

improve the arg value change detector

* add a "nocache" arg to system

* add "cache" arg to system

* small polish

add a sniffer for the XWD format (#4613)

otherwise we mistake it for TGA

see #4612

Update ChangeLog ahead of v8.17.1 (#4603)

oop ahem revert the -rc1 in meson version