Fix: GPv3: Memory leaks from Grease Pencil undo steps

The `StepObject` stores customdata but relies on C++ object destruction.
CustomData is a C struct and doesn't get freed automatically, the
StepObject needs a destructor that takes care of the owned custom data.

Pull Request: https://projects.blender.org/blender/blender/pulls/119125

source/blender/editors/grease_pencil/intern/grease_pencil_undo.cc

@@ -290,6 +290,11 @@ class StepObject {
   }
 
  public:
+  ~StepObject()
+  {
+    CustomData_free(&layers_data_, layers_num_);
+  }
+
   void encode(Object *ob, StepEncodeStatus &encode_status)
   {
     const GreasePencil &grease_pencil = *static_cast<GreasePencil *>(ob->data);