Mir's demo-shell is used for showing off the possibilities of this Wayland competitor and experimenting with Mir code. The demo-shell now has its own custom renderer. This renderer right now doesn't do much over its existing GL renderer, but evidently there's plans to do more going forward.
Daniel van Vugt wrote in the Bzr revision introducing the custom renderer for demo-shell, "This will be used for future window management/decoration experiments. But for now, it just sets a custom background colour for demo-shell (grey)."