improve pngsave compression for palette images
improve pngsave compression for palette images

set pngsave default filter to none for palette images, since they usually
have low local correlation

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