Basic Imgui integration

Very basic and dangerous imgui integration :smiling_imp: 

Ingui is not thread safe and is built for immediate mode, so it's does not fit well with or paralelized architecture. But for the moment it'll be enought and will help me to debug some stuff.

I'll integrate it cleanly asap.

If you don't need imgui, in encourage you to comment the `#define USE_IMG` :smile: