Code review comment for lp:~kdub/mir/display-groups

I think much greater care needs to be taken by all here.

This proposal can and should be improved. The DisplayGroup is not only undesirable for desktop but downright dangerous for a desktop compositor to use. Because there's no way to do so (e.g. with Mesa) without either causing frame skipping, reduced performance or lag.

As such, please hide DisplayGroup from the common code so that desktoppers can't accidentally find themselves using it.