Demo branch does not work properly

[liux-pro]

Hi, @but0n

I want to have a quick test on this library, so I checkout demo branch in git, compile (ubuntu20.04 gcc) and run.
I got a test.ppm but it is a total black image.
It seem that it does not work properly.

Is there specific settings for me environment to compile ol3d that I missed? Or what's the best way to have a test?

Thanks~