WindowManager::add_display()/remove_display()

In one of the implementations of this interface SystemCompositorWindowManager we have:

    void add_display(mir::geometry::Rectangle const& area) override
    __attribute__((deprecated("Mir doesn't reliably call this: it is ignored. Use add_display_for_testing() instead")));

And, in this implementation, the outputs are tracked by the ActiveOutputsListener interface.

But the other implementation SystemCompositorWindowManager uses this function to track the outputs.

There's clearly something wrong with the diagnostic, one of the implementations, or several of these.