Lay on Face: flip a face pointing straight up about X, not an arbitrary axis

Laying a face that already points up down needs a half turn, and every
horizontal axis does the job. setFromTwoVectors() resolved the choice through an
SVD whose null space is the whole XY plane, so the axis followed rounding noise:
the object landed at an arbitrary angle in the plate plane, and a part that fit
the plate no longer did. Turn such a face over about X, which keeps the
footprint, and share the rotation between the gizmo and the --ground-* options
instead of duplicating it.

src/slic3r/GUI/Selection.cpp

@@ -17,6 +17,7 @@
 #include "libslic3r/Color.hpp"
 #include "libslic3r/Point.hpp"
 #include "libslic3r/Geometry.hpp"
+#include "libslic3r/LayOnFace.hpp"
 #include "slic3r/GUI/GLModel.hpp"
 #include <string>
 #include <vector>
@@ -1472,11 +1473,9 @@ void Selection::flattening_rotate(const Vec3d& normal)
 
     for (unsigned int i : m_list) {
         GLVolume& v = *(*m_volumes)[i];
-        // Normal transformed from the object coordinate space to the world coordinate space.
+        // Additional rotation to align the face normal with the down vector in the world coordinate space.
         const Geometry::Transformation& old_inst_trafo = v.get_instance_transformation();
-        const Vec3d tnormal = old_inst_trafo.get_matrix().matrix().block(0, 0, 3, 3).inverse().transpose() * normal;
-        // Additional rotation to align tnormal with the down vector in the world coordinate space.
-        const Transform3d rotation_matrix = Transform3d(Eigen::Quaterniond().setFromTwoVectors(tnormal, -Vec3d::UnitZ()));
+        const Transform3d rotation_matrix = lay_on_face_rotation(normal, old_inst_trafo.get_matrix_no_offset());
         v.set_instance_transformation(old_inst_trafo.get_offset_matrix() * rotation_matrix * old_inst_trafo.get_matrix_no_offset());
     }