stack: Add some trace instrumentation

The stack and stack tracker tend to cause missed frames from time to
time, especially when there are many open windows. Add some
instrumentation to make it this easily verifiable when profiling.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1616>

src/core/stack.c

@@ -78,6 +78,8 @@ on_stack_changed (MetaStack *stack)
   GArray *hidden_stack_ids;
   GList *sorted;
 
+  COGL_TRACE_BEGIN_SCOPED (StackChanged, "Stack changed");
+
   meta_topic (META_DEBUG_STACK, "Syncing window stack to server");
 
   all_root_children_stacked = g_array_new (FALSE, FALSE, sizeof (uint64_t));
@@ -259,6 +261,8 @@ meta_stack_changed (MetaStack *stack)
   if (stack->freeze_count > 0)
     return;
 
+  COGL_TRACE_BEGIN_SCOPED (MetaStackChangedSort, "Stack: Changed");
+
   stack_ensure_sorted (stack);
   g_signal_emit (stack, signals[CHANGED], 0);
 }