libosmesa fallback not working

[fidoriel]

Hey,

great lib. Makes my project possible in the first place.
But the libosmesa fallback is not working within a docker container and I do not want to bother with x11 or wayland or any frame buffer tool.

To fix this on a fork, I just removed all code used for other render backends: fidoriel@0b2ab31

This enforces libosmesa and fails if this fails. Because this fix is working means, that libosmesa is working, but the fallback does not.

May be related to
#55
#27