Support GPU offloading using NVidia drivers

When using the open source graphics drivers, DRI_PRIME=1 can be set to enable GPU render offloading of an application.

The NVidia drivers instead use __NV_PRIME_RENDER_OFFLOAD=1 and __GLX_VENDOR_LIBRARY_NAME=nvidia [1].

Add these variables when launching through GNOME to enable offloading using NVidia drivers.