vips 7.40.9

Includes a bunch of fixes, thanks @jcupitt!

https://github.com/jcupitt/libvips/issues/172
https://github.com/jcupitt/libvips/issues/176
https://github.com/jcupitt/libvips/issues/177
https://github.com/jcupitt/libvips/issues/178
https://github.com/jcupitt/libvips/issues/180