Fix #129024: GPv3: Renaming a vertex group removes group assignment

GPv3 stores the vertex group inside `CurvesGeometry`.
When the object level vertex group name is changed, we have to loop over
the vertex group list of each drawing. If a matching name is
found, copy the new name to the vertex group in `CurvesGeometry`. A separate
function is created to handle this: `BKE_grease_pencil_vgroup_name_update`.

Pull Request: https://projects.blender.org/blender/blender/pulls/129038

source/blender/blenkernel/BKE_deform.hh

@@ -92,6 +92,7 @@ int *BKE_object_defgroup_flip_map_single(const Object *ob,
 int BKE_object_defgroup_flip_index(const Object *ob, int index, bool use_default);
 int BKE_object_defgroup_name_index(const Object *ob, blender::StringRef name);
 void BKE_object_defgroup_unique_name(bDeformGroup *dg, Object *ob);
+void BKE_object_defgroup_set_name(bDeformGroup *dg, Object *ob, const char *new_name);
 
 MDeformWeight *BKE_defvert_find_index(const MDeformVert *dv, int defgroup);
 /**

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -1047,6 +1047,8 @@ void BKE_grease_pencil_copy_layer_group_parameters(
 void BKE_grease_pencil_nomain_to_grease_pencil(GreasePencil *grease_pencil_src,
                                                GreasePencil *grease_pencil_dst);
 
+void BKE_grease_pencil_vgroup_name_update(Object *ob, const char *old_name, const char *new_name);
+
 void BKE_grease_pencil_data_update(Depsgraph *depsgraph, Scene *scene, Object *object);
 void BKE_grease_pencil_duplicate_drawing_array(const GreasePencil *grease_pencil_src,
                                                GreasePencil *grease_pencil_dst);

source/blender/blenkernel/intern/deform.cc

@@ -25,6 +25,7 @@
 #include "BLI_listbase.h"
 #include "BLI_math_vector.h"
 #include "BLI_string.h"
+#include "BLI_string_utf8.h"
 #include "BLI_string_utils.hh"
 #include "BLI_utildefines.h"
 
@@ -754,6 +755,18 @@ void BKE_object_defgroup_unique_name(bDeformGroup *dg, Object *ob)
   BLI_uniquename_cb(defgroup_unique_check, &data, DATA_("Group"), '.', dg->name, sizeof(dg->name));
 }
 
+void BKE_object_defgroup_set_name(bDeformGroup *dg, Object *ob, const char *new_name)
+{
+  std::string old_name = dg->name;
+  STRNCPY_UTF8(dg->name, new_name);
+  BKE_object_defgroup_unique_name(dg, ob);
+
+  if (ob->type == OB_GREASE_PENCIL) {
+    /* Update vgroup names stored in CurvesGeometry */
+    BKE_grease_pencil_vgroup_name_update(ob, old_name.c_str(), dg->name);
+  }
+}
+
 float BKE_defvert_find_weight(const MDeformVert *dvert, const int defgroup)
 {
   MDeformWeight *dw = BKE_defvert_find_index(dvert, defgroup);

source/blender/blenkernel/intern/grease_pencil.cc

@@ -1890,6 +1890,21 @@ void BKE_grease_pencil_nomain_to_grease_pencil(GreasePencil *grease_pencil_src,
   BKE_id_free(nullptr, grease_pencil_src);
 }
 
+void BKE_grease_pencil_vgroup_name_update(Object *ob, const char *old_name, const char *new_name)
+{
+  using namespace blender::bke::greasepencil;
+  GreasePencil &grease_pencil = *static_cast<GreasePencil *>(ob->data);
+  for (GreasePencilDrawingBase *base : grease_pencil.drawings()) {
+    Drawing &drawing = reinterpret_cast<GreasePencilDrawing *>(base)->wrap();
+    CurvesGeometry &curves = drawing.strokes_for_write();
+    LISTBASE_FOREACH (bDeformGroup *, vgroup, &curves.vertex_group_names) {
+      if (strcmp(vgroup->name, old_name) == 0) {
+        STRNCPY(vgroup->name, new_name);
+      }
+    }
+  }
+}
+
 static void grease_pencil_evaluate_modifiers(Depsgraph *depsgraph,
                                              Scene *scene,
                                              Object *object,

source/blender/makesrna/intern/rna_object.cc

@@ -902,8 +902,7 @@ static void rna_VertexGroup_name_set(PointerRNA *ptr, const char *value)
   }
 
   bDeformGroup *dg = static_cast<bDeformGroup *>(ptr->data);
-  STRNCPY_UTF8(dg->name, value);
-  BKE_object_defgroup_unique_name(dg, ob);
+  BKE_object_defgroup_set_name(dg, ob, value);
 }
 
 static int rna_VertexGroup_index_get(PointerRNA *ptr)