improve colourspace guessing in icc
improve colourspace guessing in icc

icc_transform does a better job of guessing the colourspace of an image -- if
the colourspace has not been set (generic multiband), it'll look at the
image bands instead and guess from that

see https://github.com/jcupitt/libvips/issues/971

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