eglfs should warn about running on llvmpipe

Linux distros on ARM targeting the Raspberry Pi tend to make the horrible decision of shipping and defaulting to Mesa with the llvmpipe backend. This way they get OpenGL under X11, and everyone is happy, at least until they discovers that real apps (say normal Qt Quick stuff) cannot perform in this setup.

While we cannot do much about people trying to use Qt with xcb, we can mitigate the headache and confusion with eglfs somewhat: let's add a warning in case we find we are running on llvmpipe.