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.
> similar to the workaround we apply with a hidden instance of 1x1 Ogre Window (which takes up memory and only gets destroyed when OpenCS exists).

No, this is different. Created render windows are properly destroyed when Ogre::Root shuts down. Objects that you allocated yourself via new() are not. While the memory may be reclaimed by the OS after the application has shut down, this is still a memory leak and not considered acceptable.
https://stackoverflow.com/questions/273209/are-memory-leaks-ever-ok