OpenCS: Render cell markers

This is gugus's work (PR  #187).  All I did was merge in the master branch and made it compile with gcc on ubuntu.

Re-implemented using overlays.
>  I'm not sure if there is any real harm of keeping it around till OpenCS exits.

There is always harm in a memory leak.

I can't comment on your fix suggestions because I'm not too familiar with OpenCS. Maybe Zini can help out.