Add demo about imgui

Add [this](https://github.com/ocornut/imgui/blob/master/examples/example_glut_opengl2/main.cpp) demo about the use of [dear imgui](https://github.com/ocornut/imgui).

For the record, one of the comments at the beginning of the file:

> !!! If someone or something is teaching you GLUT in 2019, you are being abused. Please show some resistance. !!!

I agree, but life is tough.