Fix #103562: preserve custom normals in BM_mesh_triangulate

`BM_mesh_triangulate` is used in exporters (when the "Triangulate"
option is ON), the `Triangulate` modifier and currently also in the
`Triangulate` geometry node (even though there are plans to change this,
see !112264)

So in practice, exporters (Alembic/FBX/OBJ/Collada) were breaking
custom normals for game pipelines (unless everything was triangulated
beforehand).

This change builds upon 93c8955a72 (uses the use
`BM_custom_loop_normals_to_vector_layer` /
`BM_custom_loop_normals_from_vector_layer` pair of calls).

In the case of the `Triangulate` modifier, this had its own try at
preserving custom normals in 7d0fcaa69a  -- doing very similar
things but as an option -- this is now removed (so it is always done,
which fits into "interpolate custom data if it's there" design that we have
nowadays).

NOTE: the "Triangulate Faces" operator already did the same
Pull Request: https://projects.blender.org/blender/blender/pulls/121871

source/blender/bmesh/tools/bmesh_triangulate.cc

@@ -90,6 +90,8 @@ void BM_mesh_triangulate(BMesh *bm,
   MemArena *pf_arena;
   Heap *pf_heap;
 
+  BM_custom_loop_normals_to_vector_layer(bm);
+
   pf_arena = BLI_memarena_new(BLI_POLYFILL_ARENA_SIZE, __func__);
 
   if (ngon_method == MOD_TRIANGULATE_NGON_BEAUTY) {
@@ -150,6 +152,8 @@ void BM_mesh_triangulate(BMesh *bm,
 
   BLI_memarena_free(pf_arena);
 
+  BM_custom_loop_normals_from_vector_layer(bm, false);
+
   if (ngon_method == MOD_TRIANGULATE_NGON_BEAUTY) {
     BLI_heap_free(pf_heap, nullptr);
   }

source/blender/editors/mesh/editmesh_tools.cc

@@ -5500,8 +5500,6 @@ static int edbm_quads_convert_to_tris_exec(bContext *C, wmOperator *op)
     BMOIter oiter;
     BMFace *f;
 
-    BM_custom_loop_normals_to_vector_layer(em->bm);
-
     EDBM_op_init(em,
                  &bmop,
                  op,
@@ -5526,8 +5524,6 @@ static int edbm_quads_convert_to_tris_exec(bContext *C, wmOperator *op)
       continue;
     }
 
-    BM_custom_loop_normals_from_vector_layer(em->bm, false);
-
     EDBMUpdate_Params params{};
     params.calc_looptris = true;
     params.calc_normals = false;

source/blender/makesdna/DNA_modifier_types.h

@@ -1898,12 +1898,11 @@ typedef struct TriangulateModifierData {
 } TriangulateModifierData;
 
 /** #TriangulateModifierData.flag */
-enum {
 #ifdef DNA_DEPRECATED_ALLOW
+enum {
   MOD_TRIANGULATE_BEAUTY = (1 << 0), /* deprecated */
-#endif
-  MOD_TRIANGULATE_KEEP_CUSTOMLOOP_NORMALS = 1 << 1,
 };
+#endif
 
 /** #TriangulateModifierData.ngon_method triangulate method (N-gons). */
 enum {

source/blender/makesrna/intern/rna_modifier.cc

@@ -6597,16 +6597,6 @@ static void rna_def_modifier_triangulate(BlenderRNA *brna)
       "Triangulate only polygons with vertex count greater than or equal to this number");
   RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
-  prop = RNA_def_property(srna, "keep_custom_normals", PROP_BOOLEAN, PROP_NONE);
-  RNA_def_property_boolean_sdna(prop, nullptr, "flag", MOD_TRIANGULATE_KEEP_CUSTOMLOOP_NORMALS);
-  RNA_def_property_ui_text(
-      prop,
-      "Keep Normals",
-      "Try to preserve custom normals.\n"
-      "Warning: Depending on chosen triangulation method, "
-      "shading may not be fully preserved, \"Fixed\" method usually gives the best result here");
-  RNA_def_property_update(prop, 0, "rna_Modifier_update");
-
   RNA_define_lib_overridable(false);
 }
 

source/blender/modifiers/intern/MOD_triangulate.cc

@@ -43,15 +43,6 @@ static Mesh *triangulate_mesh(Mesh *mesh,
   cd_mask_extra.emask = CD_MASK_ORIGINDEX;
   cd_mask_extra.pmask = CD_MASK_ORIGINDEX;
 
-  bool keep_clnors = (flag & MOD_TRIANGULATE_KEEP_CUSTOMLOOP_NORMALS) != 0;
-
-  if (keep_clnors) {
-    void *data = CustomData_add_layer(
-        &mesh->corner_data, CD_NORMAL, CD_CONSTRUCT, mesh->corners_num);
-    memcpy(data, mesh->corner_normals().data(), mesh->corner_normals().size_in_bytes());
-    cd_mask_extra.lmask |= CD_MASK_NORMAL;
-  }
-
   BMeshCreateParams bmesh_create_params{};
   BMeshFromMeshParams bmesh_from_mesh_params{};
   bmesh_from_mesh_params.calc_face_normal = true;
@@ -66,13 +57,6 @@ static Mesh *triangulate_mesh(Mesh *mesh,
   result = BKE_mesh_from_bmesh_for_eval_nomain(bm, &cd_mask_extra, mesh);
   BM_mesh_free(bm);
 
-  if (keep_clnors) {
-    float(*corner_normals)[3] = static_cast<float(*)[3]>(
-        CustomData_get_layer_for_write(&result->corner_data, CD_NORMAL, result->corners_num));
-    BKE_mesh_set_custom_normals(result, corner_normals);
-    CustomData_free_layers(&result->corner_data, CD_NORMAL, result->corners_num);
-  }
-
   return result;
 }