Fix #108273: Mirror modifier resets copied custom normals

The "no custom" normal used to be stored inside the custom normal space
struct, now it's stored separately. Before the normal was modified, but
not the one in the normal space struct. Fixed by storing the original
before modification in a temporary variable.

source/blender/blenkernel/intern/mesh_mirror.cc

@@ -430,12 +430,14 @@ Mesh *BKE_mesh_mirror_apply_mirror_on_axis_for_modifier(MirrorModifierData *mmd,
         if (j > src_poly.start()) {
           mirrorj += result_polys[mirror_i].size() - (j - src_poly.start());
         }
+
+        const blender::float3 orig_normal = loop_normals[mirrorj];
         copy_v3_v3(loop_normals[mirrorj], loop_normals[j]);
         mul_m4_v3(mtx_nor, loop_normals[mirrorj]);
 
         const int space_index = lnors_spacearr.corner_space_indices[mirrorj];
         blender::bke::mesh::lnor_space_custom_normal_to_data(&lnors_spacearr.spaces[space_index],
-                                                             loop_normals[mirrorj],
+                                                             orig_normal,
                                                              loop_normals[mirrorj],
                                                              clnors[mirrorj]);
       }