swap abs() for fabs() in composite
swap abs() for fabs() in composite

The non-vector fallback path had abs(double). Thanks brcarp.

See https://github.com/libvips/libvips/issues/1572

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