Code review comment for lp:~kdub/mir/fix-1299977-take3

P.S. If you want to enable overlay optimizations by default, then you should probably be focusing more on how the Ubuntu touch Qt shell needs it.

In the least you can overlay full-screen surfaces (on the host server when nesting) right now using the bypass detection algorithm. Then worry about enabling hardware overlays in the nested server later. So we can get most of the performance benefits of overlays turned on right now without worrying at all about anything to to with demo shell.