Cycles: Fix shading with autosmooth and custom normals

New logic of split_faces was leaving mesh in a proper state
from Blender's point of view, but Cycles wanted loop normals
to be "flushed" to vertex normals.

Now we do such a flush from Cycles side again, so we don't
leave bad meshes behind.

Thanks Bastien for assistance here!

intern/cycles/blender/blender_mesh.cpp

@@ -777,6 +777,15 @@ static void create_mesh(Scene *scene,
 			int shader = clamp(f->material_index(), 0, used_shaders.size()-1);
 			bool smooth = f->use_smooth() || use_loop_normals;
 
+			if(use_loop_normals) {
+				BL::Array<float, 12> loop_normals = f->split_normals();
+				for(int i = 0; i < n; i++) {
+					N[vi[i]] = make_float3(loop_normals[i * 3],
+					                       loop_normals[i * 3 + 1],
+					                       loop_normals[i * 3 + 2]);
+				}
+			}
+
 			/* Create triangles.
 			 *
 			 * NOTE: Autosmooth is already taken care about.

intern/cycles/blender/blender_util.h

@@ -79,7 +79,7 @@ static inline BL::Mesh object_to_mesh(BL::BlendData& data,
 				me.calc_normals_split();
 			}
 			else {
-				me.split_faces();
+				me.split_faces(false);
 			}
 		}
 		if(subdivision_type == Mesh::SUBDIVISION_NONE) {

source/blender/blenkernel/BKE_mesh.h

@@ -131,7 +131,7 @@ bool BKE_mesh_uv_cdlayer_rename(struct Mesh *me, const char *old_name, const cha
 
 float (*BKE_mesh_vertexCos_get(const struct Mesh *me, int *r_numVerts))[3];
 
-void BKE_mesh_split_faces(struct Mesh *mesh);
+void BKE_mesh_split_faces(struct Mesh *mesh, bool free_loop_normals);
 
 struct Mesh *BKE_mesh_new_from_object(struct Main *bmain, struct Scene *sce, struct Object *ob,
                                       int apply_modifiers, int settings, int calc_tessface, int calc_undeformed);

source/blender/blenkernel/intern/mesh.c

@@ -2338,7 +2338,7 @@ static void split_faces_split_new_edges(
  * NOTE: Will leave CD_NORMAL loop data layer which is
  * used by render engines to set shading up.
  */
-void BKE_mesh_split_faces(Mesh *mesh)
+void BKE_mesh_split_faces(Mesh *mesh, bool free_loop_normals)
 {
 	const int num_polys = mesh->totpoly;
 
@@ -2392,7 +2392,9 @@ void BKE_mesh_split_faces(Mesh *mesh)
 	/* Note: after this point mesh is expected to be valid again. */
 
 	/* CD_NORMAL is expected to be temporary only. */
-	CustomData_free_layers(&mesh->ldata, CD_NORMAL, mesh->totloop);
+	if (free_loop_normals) {
+		CustomData_free_layers(&mesh->ldata, CD_NORMAL, mesh->totloop);
+	}
 
 	if (lnors_spacearr) {
 		/* Also frees new_verts/edges temp data, since we used its memarena to allocate them. */

source/blender/editors/object/object_bake_api.c

@@ -619,7 +619,7 @@ static Mesh *bake_mesh_new_from_object(Main *bmain, Scene *scene, Object *ob)
 		ED_object_editmode_load(ob);
 
 	Mesh *me = BKE_mesh_new_from_object(bmain, scene, ob, 1, 2, 0, 0);
-	BKE_mesh_split_faces(me);
+	BKE_mesh_split_faces(me, true);
 
 	return me;
 }

source/blender/makesrna/intern/rna_mesh_api.c

@@ -209,6 +209,11 @@ static void rna_Mesh_flip_normals(Mesh *mesh)
 	DAG_id_tag_update(&mesh->id, 0);
 }
 
+static void rna_Mesh_split_faces(Mesh *mesh, int free_loop_normals)
+{
+	BKE_mesh_split_faces(mesh, free_loop_normals != 0);
+}
+
 #else
 
 void RNA_api_mesh(StructRNA *srna)
@@ -240,8 +245,10 @@ void RNA_api_mesh(StructRNA *srna)
 	func = RNA_def_function(srna, "free_normals_split", "rna_Mesh_free_normals_split");
 	RNA_def_function_ui_description(func, "Free split vertex normals");
 
-	func = RNA_def_function(srna, "split_faces", "BKE_mesh_split_faces");
+	func = RNA_def_function(srna, "split_faces", "rna_Mesh_split_faces");
 	RNA_def_function_ui_description(func, "Split faces based on the edge angle");
+	RNA_def_boolean(func, "free_loop_normals", 1, "Free Loop Notmals",
+	                "Free loop normals custom data layer");
 
 	func = RNA_def_function(srna, "calc_tangents", "rna_Mesh_calc_tangents");
 	RNA_def_function_flag(func, FUNC_USE_REPORTS);