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-main.c

@@ -73,6 +73,7 @@ typedef struct _MetaContextMainOptions
 #ifdef HAVE_NATIVE_BACKEND
   GList *virtual_monitor_infos;
 #endif
+  char *trace_file;
 } MetaContextMainOptions;
 
 struct _MetaContextMain
@@ -298,6 +299,10 @@ meta_context_main_configure (MetaContext   *context,
         context_main->options.sm.client_id = g_strdup (desktop_autostart_id);
     }
 
+#ifdef HAVE_PROFILER
+  meta_context_set_trace_file (context, context_main->options.trace_file);
+#endif
+
   g_unsetenv ("DESKTOP_AUTOSTART_ID");
 
   return TRUE;
@@ -662,6 +667,12 @@ meta_context_main_add_option_entries (MetaContextMain *context_main)
       N_("Run with X11 backend")
     },
 #endif
+    {
+      "profile", 0, 0, G_OPTION_ARG_FILENAME,
+      &context_main->options.trace_file,
+      N_("Profile performance using trace instrumentation"),
+      "FILE"
+    },
     { NULL }
   };