more optional output arg support
more optional output arg support

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