monitor-manager: fix output ids returned by GetResources

Output ID is set equal to 'i' later in the loop. But 'i' was never
incremented, so all outputs were getting the same ID (equal to
the number of CRTCs, because 'i' was reused from the previous loop).


(cherry picked from commit 23c3f8bb18)

src/backends/meta-monitor-manager.c

@@ -1032,7 +1032,7 @@ meta_monitor_manager_handle_get_resources (MetaDBusDisplayConfig *skeleton,
                              NULL /* properties */);
     }
 
-  for (l = combined_outputs; l; l = l->next)
+  for (l = combined_outputs, i = 0; l; l = l->next, i++)
     {
       MetaOutput *output = l->data;
       GVariantBuilder crtcs, modes, clones, properties;