Backends: SDL2,SDL3: added ImGui_ImplSDL2_InitForOther() / ImGui_ImplSDL3_InitForOther().

e.g. WebGPU users use a similarly named functions from GLFW backend.

backends/imgui_impl_sdl3.cpp

@@ -413,6 +413,11 @@ bool ImGui_ImplSDL3_InitForSDLRenderer(SDL_Window* window, SDL_Renderer* rendere
     return ImGui_ImplSDL3_Init(window, renderer);
 }
 
+bool ImGui_ImplSDL2_InitForOther(SDL_Window* window)
+{
+    return ImGui_ImplSDL2_Init(window, nullptr);
+}
+
 void ImGui_ImplSDL3_Shutdown()
 {
     ImGui_ImplSDL3_Data* bd = ImGui_ImplSDL3_GetBackendData();