add op invalidate stuff

still need to test and link to cache
fix up operation invalidate

cache drop next
cache drops operations on invalidate

we can now enable the vips8 operation cache in nip2, woo!
more fixups to pass testsuite
update notes
oops, missed another NULL
stop image abuse in labelregions

we were marking as image as changing by calling vips_image_readwrite(),
but the cache system didn't uncache it
final clean-up