apply --centre to upsize as well as down
apply --centre to upsize as well as down

we were just using the --centre flag in resize to influence downsize
behaviour -- use it to set upsize as well

see https://github.com/jcupitt/libvips/issues/705

lovell/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips