Code review comment for lp:~andreas-pokorny/mir/allow-transparent-server-buffers

pixel_format_utils.cpp could eventually be replaced by the proposal in bug 1236254. But they're not conflicting ideas so that's OK.

I haven't followed this through yet, but please read the existing discussion --> bug 1256702
The important point is to ensure that translucency is optional, _never_ used on a real shell, and only used in temporary things like greeters.