Cleanup: Use const qualifier in modifier data copy

Fix possible overflow of Modifier UUID

The code prior this change was re-generating modifier's session UUID
prior to copying this id from the source. This approach has a higher
risk of modifiers session UUID to overflow and start colliding with
existing modifiers.

This change makes it so that modifier copy does not re-generated the
session UUID unless it is needed.

Differential Revision: https://developer.blender.org/D14125

source/blender/blenkernel/BKE_modifier.h

@@ -409,6 +409,8 @@ void BKE_modifier_session_uuid_generate(struct ModifierData *md);
 
 bool BKE_modifier_unique_name(struct ListBase *modifiers, struct ModifierData *md);
 
+struct ModifierData *BKE_modifier_copy_ex(const struct ModifierData *md, int flag);
+
 /**
  * Callback's can use this to avoid copying every member.
  */

source/blender/blenkernel/intern/modifier.c

@@ -131,7 +131,7 @@ void BKE_modifier_panel_expand(ModifierData *md)
 
 /***/
 
-ModifierData *BKE_modifier_new(int type)
+static ModifierData *modifier_allocate_and_init(int type)
 {
   const ModifierTypeInfo *mti = BKE_modifier_get_info(type);
   ModifierData *md = MEM_callocN(mti->structSize, mti->structName);
@@ -152,6 +152,13 @@ ModifierData *BKE_modifier_new(int type)
     mti->initData(md);
   }
 
+  return md;
+}
+
+ModifierData *BKE_modifier_new(int type)
+{
+  ModifierData *md = modifier_allocate_and_init(type);
+
   BKE_modifier_session_uuid_generate(md);
 
   return md;
@@ -315,6 +322,16 @@ void BKE_modifiers_foreach_tex_link(Object *ob, TexWalkFunc walk, void *userData
   }
 }
 
+ModifierData *BKE_modifier_copy_ex(const ModifierData *md, int flag)
+{
+  ModifierData *md_dst = modifier_allocate_and_init(md->type);
+
+  BLI_strncpy(md_dst->name, md->name, sizeof(md_dst->name));
+  BKE_modifier_copydata_ex(md, md_dst, flag);
+
+  return md_dst;
+}
+
 void BKE_modifier_copydata_generic(const ModifierData *md_src,
                                    ModifierData *md_dst,
                                    const int UNUSED(flag))

source/blender/blenkernel/intern/object.cc

@@ -1599,9 +1599,7 @@ bool BKE_object_modifier_stack_copy(Object *ob_dst,
       continue;
     }
 
-    ModifierData *md_dst = BKE_modifier_new(md_src->type);
-    BLI_strncpy(md_dst->name, md_src->name, sizeof(md_dst->name));
-    BKE_modifier_copydata_ex(md_src, md_dst, flag_subdata);
+    ModifierData *md_dst = BKE_modifier_copy_ex(md_src, flag_subdata);
     BLI_addtail(&ob_dst->modifiers, md_dst);
   }