don't modify the operation cache in a callback
don't modify the operation cache in a callback

oops, we were removing things from the operation hash during a traverse

jcupitt/libvips,wishabi/libvips,mcanthony/libvips,wishabi/libvips,jcupitt/libvips,lovell/libvips,leiyangyou/libvips,mcanthony/libvips,wishabi/libvips,jcupitt/libvips,mcanthony/libvips,leiyangyou/libvips,wishabi/libvips,mcanthony/libvips,leiyangyou/libvips,jcupitt/libvips,lovell/libvips,leiyangyou/libvips,lovell/libvips,lovell/libvips