fix im_addgnoise
fix im_addgnoise

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