Frame: refine critical lens regions

tests/test_schwarzschild.c

@@ -64,6 +64,7 @@ int main(void) {
   const RefinementConfig refinement = {.max_level = 1,
                                        .angle_absolute_rad = 1e-5,
                                        .angle_relative = 1e-5,
+                                       .jacobian_minimum = 1e-3,
                                        .min_edge_pixels = 1.0,
                                        .min_area_pixels2 = 1.0};
   if (frame_lens_mesh_build_coarse(&mesh, 48, 48, 24, 40.0) ||