We fork off twice once with and once without DIR_PRIME=1 set. We
then use the vendor string to determine what GPU to use.
We prefer (in order)
1) AMDGPU/AMDGPU-PRO/NVidia non-free driver
2) Intel driver
3) Nouveau
4) Software rendering
If a driver can't be detected it will default to DRI_PRIME=0
