Code review comment for lp:~robertcarr/mir/alpha-for-nested-servers

I'm told the Unity8 shell is implemented using alpha right now. Partly because Mir didn't have surface resizing implemented (yet). So I guess as soon as we can, we should make sure Unity8 uses non-alpha pixel formats (where possible). XMir already does.

As for the whole nested display... I think we need to default to opaque surface formats for performance, not:
38 + egl_pixel_format{find_transparent_surface_format(*connection)},
Although obviously to suit greeter needs, that should be configurable as transparent. Which is fine if it only occurs while the greeter is visible.

I'm basing this on assumptions about how nesting actually works, so need some guidance as to whether my concerns are realistic at all... So if the greeter's on top of a session, is it composited with the existing compositing logic or something new?