cogl/trace: Make COGL_TRACE_DESCRIBE append

Sometimes it's useful to combine the description from multiple places.
For example, a subsequent commit will add the output name as the first
thing to the frame clock span descriptions, leaving the rest of the
description with complex checks unchanged.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3417>

cogl/cogl/cogl-trace.c

@@ -342,6 +342,14 @@ void
 cogl_trace_describe (CoglTraceHead *head,
                      const char    *description)
 {
+  if (head->description)
+    {
+      char *old_description = head->description;
+      head->description =
+        g_strdup_printf ("%s, %s", old_description, description);
+      g_free (old_description);
+    }
+  else
     head->description = g_strdup (description);
 }