message: Make it possible to use SetNextWindowPos() on a child window. Useful internally.
diff: @@ -4295,7 +4295,10 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
  window->SizeFull = CalcSizeFullWithConstraint(window, window->SizeFull);
  window->Size = window->Collapsed ? window->TitleBarRect().GetSize() : window->SizeFull;
  if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup))
  + {
  + IM_ASSERT(window_size_set_by_api); // Submitted by BeginChild()
  window->Size = window->SizeFull;
  + }
  // SCROLLBAR STATUS
  