context: Start persistent profiling via command line argument

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>

src/core/meta-context-private.h

@@ -86,6 +86,9 @@ gboolean meta_context_is_x11_sync (MetaContext *context);
 #ifdef HAVE_PROFILER
 MetaProfiler *
 meta_context_get_profiler (MetaContext *context);
+
+void meta_context_set_trace_file (MetaContext *context,
+                                  const char  *trace_file);
 #endif
 
 #endif /* META_CONTEXT_PRIVATE_H */