Menus: Implement BeginMenu() appending to existing menu when executed with same ID multiple times. (#1207)

imgui.cpp

@@ -3623,6 +3623,7 @@ void ImGui::NewFrame()
     g.FrameCount += 1;
     g.TooltipOverrideCount = 0;
     g.WindowsActiveCount = 0;
+    g.RenderedMenusId.resize(0);
 
     // Setup current font and draw list shared data
     g.IO.Fonts->Locked = true;