util: Add INPUT debug topic

Allow debugging input issues more easily by adding an INPUT debug topic.
Currently there are only debug messages for the native backend.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1065

src/core/util.c

@@ -335,6 +335,8 @@ topic_name (MetaDebugTopic topic)
       return "EDGE_RESISTANCE";
     case META_DEBUG_DBUS:
       return "DBUS";
+    case META_DEBUG_INPUT:
+      return "INPUT";
     case META_DEBUG_VERBOSE:
       return "VERBOSE";
     }