update Dear ImGui to 1.89.9

this is part of an upgrade process to the latest version of Dear ImGui
it will take a while as I am stepping through individual versions due
to the heavy modifications we've made for Furnace

extern/imgui_patched/backends/imgui_impl_sdl2.cpp

@@ -557,6 +557,11 @@ bool ImGui_ImplSDL2_InitForSDLRenderer(SDL_Window* window, SDL_Renderer* rendere
     return ImGui_ImplSDL2_Init(window, renderer, nullptr);
 }
 
+bool ImGui_ImplSDL2_InitForOther(SDL_Window* window)
+{
+    return ImGui_ImplSDL2_Init(window, nullptr, nullptr);
+}
+
 void ImGui_ImplSDL2_Shutdown()
 {
     ImGui_ImplSDL2_Data* bd = ImGui_ImplSDL2_GetBackendData();