Prevent non-critical race condition in op cache

First thread to add an operation 'wins'
Merge pull request #181 from lovell/prevent-opcache-race-7.40

Allow op-cache winner in non-critical race condition
tiny cache clean-up
Merge branch '7.40'
oop, dropped a * on the floor
Merge branch '7.40'
not quite there
oops, double allocate

thanks Lovell
Merge branch '7.40'
working!
Merge branch 'colour-fix'
update notes
start conversion tests
make arrayimage ref/unref
sync
sync
Merge branch 'master' of github.com:jcupitt/libvips

Conflicts:
	TODO
fix up deCMC again
fixed extra band handling, again
test extra band handling in dE