message: Fixed old Begin() calling SetNextWindowBgAlpha() with negative values.
diff: @@ -5018,6 +5018,7 @@ bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_first_use,
  ImGui::SetNextWindowSize(size_first_use, ImGuiCond_FirstUseEver);
  // Old API feature: override the window background alpha with a parameter.
  + if (bg_alpha_override >= 0.0f)
  ImGui::SetNextWindowBgAlpha(bg_alpha_override);
  return ImGui::Begin(name, p_open, flags);
  