argh
argh
backport threaded tile cache
clean up after fix

threaded tile cache seems a good solution --- clean up and get ready to
merge back into master
7.30.7 release
Merge remote-tracking branch 'origin/7.30'

Conflicts:
	libvips/conversion/sequential.c
	libvips/conversion/tilecache.c
	libvips/deprecated/im_tile_cache.c
	libvips/deprecated/vips7compat.c
	po/vips7.pot