Fix SDL2 headless mode requiring XServer

[rk1a]

While usage of the SDL2 offscreen driver works with machines that run a XServer instance (which is a type of window display)
it does not seem to work with display-less servers (e.g. in google colab https://colab.research.google.com/drive/1KDvWgmhV445JB73WRqGijkegDVlY7e6p?usp=sharing)
Xvfb can be used to create a virtual XServer to work around that for now.
The question is: is it possible to build a headless version that does not need a XServer?