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

At least one way springs to mind in which Android can be supported without DisplayGroup:
Implement a "MultiDisplayBuffer" containing multiple displays attached to a single DisplayBufferCompositor. That way they are composited together and posted together. And the code classes don't need modification.

Alternatively, please consider other ways to avoid touching so much common code. As it stands now, this increases the risk of short and long term regressions (confusion by future developers). And it scares me. We have the ability to pause and try to think of better solutions still.