Persistent profiling was started via an env var, but that's rather hard
to discover and remember without grepping; change to use a command line
argument.
The profiler is started early, even during (though late in)
configuration, but configuration should ideally be instant and pointless
to configure.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2998>
It's not really a backend thing, and we'll want to profile e.g. loading
the backend too, so create it very early and destroy it very late and
let MetaContextMain own it.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2678>
