Fix T86170: Memory leak clearing the Python instance for COW id data

As Python can access COW ID's, ensure it's instance is kept on update.

This could happen when the "Use Self" argument was enabled for a driver.

source/blender/depsgraph/intern/eval/deg_eval_runtime_backup.cc

@@ -33,6 +33,7 @@ namespace blender::deg {
 
 RuntimeBackup::RuntimeBackup(const Depsgraph *depsgraph)
     : have_backup(false),
+      id_data({.py_instance = nullptr}),
       animation_backup(depsgraph),
       scene_backup(depsgraph),
       sound_backup(depsgraph),
@@ -51,6 +52,8 @@ void RuntimeBackup::init_from_id(ID *id)
   }
   have_backup = true;
 
+  id_data.py_instance = id->py_instance;
+
   animation_backup.init_from_id(id);
 
   const ID_Type id_type = GS(id->name);
@@ -89,6 +92,8 @@ void RuntimeBackup::restore_to_id(ID *id)
     return;
   }
 
+  id->py_instance = id_data.py_instance;
+
   animation_backup.restore_to_id(id);
 
   const ID_Type id_type = GS(id->name);

source/blender/depsgraph/intern/eval/deg_eval_runtime_backup.h

@@ -58,6 +58,11 @@ class RuntimeBackup {
    * copy-on-write mechanism. */
   bool have_backup;
 
+  /* Struct members of the ID pointer. */
+  struct {
+    void *py_instance;
+  } id_data;
+
   AnimationBackup animation_backup;
   SceneBackup scene_backup;
   SoundBackup sound_backup;