Delete resources in XdgOutput teardown

Generated code typically cleans up resource objects when the client
calls the relevant destructor.

In multi-cast mode our wrapper can be deleted at any time, whilst a
client resource exists any existing resources that have a pending
message will then crash the compositor.

Deleting resources ahead of time also resolves this. calls to this
resource will no-op. The zxdg_output_v1::destroy_func will then be
called to delete the Resource object.