Remove the ability to launch as a different user, which we don't
need because we're spawned by gdm or by the user manually on the
command line.
At the same time, require an active local session, and remove
the ability to run from anywhere by being in the right user group
(which automatically gives you root-like privileges)
https://bugzilla.gnome.org/show_bug.cgi?id=705861
To run mutter as a display server, one needs to acquire and
release the DRM master, which is only possible for root, so
we take advantage of weston-launch, a small setuid helper binary
written for the weston project. We import our own slightly
modified copy of it, because weston-launch only launches weston,
for security reasons.
https://bugzilla.gnome.org/show_bug.cgi?id=705861
