Code review comment for lp:~kdub/mir/split-platform-into-rendering-and-display

I am not sure how these two can be completely independent. E.g. buffer allocator is on the renderer side, but for overlays e.g., the allocator needs to know things about display. Perhaps there will need to be a common base class or an interface between the two. Can't see that far into the future. I guess, we can start here and add things as needed.

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